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Using GPS to mark a line in the woods Property line marking

#1 User is offline   PastorJon 

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Posted 25 June 2008 - 05:34 AM

How accurate do you believe one could mark a 3000ft long property line in the woods using only a GPS?

Let's say that we knew for sure where the two corners of the property were, and went to get a reading at those two points. Then, we took them back to the computer and used Google Earth (or something) to draw a line... and then take coordinates off the points in the line, and then go back to the woods to draw the line.

Would this work? Would it be accurate within a few feet? If both property owners agreed to do this (instead of hiring a surveyor), would it be fairly suitable?

Are there any tips/tricks anyone would offer on how to do this?

#2 User is offline   julianh 

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Posted 25 June 2008 - 06:00 AM

 PastorJon, on Jun 25 2008, 05:34 AM, said:

How accurate do you believe one could mark a 3000ft long property line in the woods using only a GPS?

Let's say that we knew for sure where the two corners of the property were, and went to get a reading at those two points. Then, we took them back to the computer and used Google Earth (or something) to draw a line... and then take coordinates off the points in the line, and then go back to the woods to draw the line.

Would this work? Would it be accurate within a few feet? If both property owners agreed to do this (instead of hiring a surveyor), would it be fairly suitable?

Are there any tips/tricks anyone would offer on how to do this?

While you could create a string of way-points along the line, a better way to mark out a straight line between two points is to navigate to the first point, then set a course for the second point. Use the off-course indicator (available on all Garmin hand-helds, I think, as well as most other hand-held GPSrs) to tell you how far you are to the left or right of your intended straight line course. That way, you can accurately walk the whole line, with an accuracy limited only by the accuracy that your GPSr can attain under the conditions, not just the finite number of points you have set up.

Hope this helps!

#3 User is offline   PastorJon 

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Posted 25 June 2008 - 06:06 AM

 julianh, on Jun 25 2008, 06:00 AM, said:


While you could create a string of way-points along the line, a better way to mark out a straight line between two points is to navigate to the first point, then set a course for the second point. Use the off-course indicator (available on all Garmin hand-helds, I think, as well as most other hand-held GPSrs) to tell you how far you are to the left or right of your intended straight line course. That way, you can accurately walk the whole line, with an accuracy limited only by the accuracy that your GPSr can attain under the conditions, not just the finite number of points you have set up.

Hope this helps!


Thanks... I guess using the "GoTo" feature would be like creating a string of infinite waypoints, huh?

I assume that a High-Sensitivity receiver would be an absolute requirement for this...

How about compass? Are there models that have a built in compass (not just a GPS-based compass that only works when you're moving)?

#4 User is offline   PastorJon 

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Posted 25 June 2008 - 06:26 AM

Would someone evaluate this plan for marking a line:

1) Go to corner A and get the coordinates
2) Go to corner B and get the coordinates
3) Use the GPS to "GoTo" A from B, marking trees with ribbon as we go.
4) Use the GPS to "GoTo" B from A, marking trees with (a different color?) ribbon as we go.
5) Walk from A to B, averaging the ribbon lines (if needed), marking with paint, and removing the ribbons.

How would you improve on this? Alternatively, is there someone who wants to say, "Hey...this is a bad idea... go hire a surveyor"? ;)

#5 User is offline   O-Mega 

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Posted 25 June 2008 - 06:44 AM

 PastorJon, on Jun 25 2008, 09:26 AM, said:

Would someone evaluate this plan for marking a line:

1) Go to corner A and get the coordinates
2) Go to corner B and get the coordinates
3) Use the GPS to "GoTo" A from B, marking trees with ribbon as we go.
4) Use the GPS to "GoTo" B from A, marking trees with (a different color?) ribbon as we go.
5) Walk from A to B, averaging the ribbon lines (if needed), marking with paint, and removing the ribbons.

How would you improve on this? Alternatively, is there someone who wants to say, "Hey...this is a bad idea... go hire a surveyor"? ;)

When dealing with property I would suggest a surveyor. I know my GPS can be accurate enough to find a micro in the woods, but I would not trust it to do survey work. The accuracy can fluctuate in different spots along the way making a straight line very difficult to navigate. If you are going to try this I would suggest getting the azimuth between the two points and using a handheld compass and one person shooting the degree while the other walk out to the furthest tree letting the first make left and right corrections and marking trees in that fashion. You can then do a back azimuth at the second corner and do it again in reverse to see if the trees match up. You will find that the line will be straighter that way and easier to average too.

#6 User is offline   JSWilson64 

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Posted 25 June 2008 - 06:52 AM

 PastorJon, on Jun 25 2008, 08:34 AM, said:

Would this work? Would it be accurate within a few feet? If both property owners agreed to do this (instead of hiring a surveyor), would it be fairly suitable?

Are there any tips/tricks anyone would offer on how to do this?

Firstly, I'd say get a GPS that can average a waypoint. Then let it average at one point for a nice long time until your accuracy was less than 10 feet (as low as possible). The other suggestions above are good for following the line.

You could mark the corners, get the GPS to tell you the bearing from one to another, then use a lensatic compass (the kind with the little window you sight thru) to sight down the line. Send someone down the line to mark trees, etc. It's only 3/5 of a mile, so eyeballing it might be enough, unless you're in dense woods.

Wet blanket time: I don't know why you're doing this (building a fence?) but there's a reason people hire surveyors. If you do it yourself like this, you're probably going to need someone (i.e., a lawyer) to draw up an agreement between the property owners. Otherwise, how do you know both parties are agreeable? And even if both parties are fine with it, the future owners may not be. When they hire a surveyor and find a fence or other improvements on their property, or find that "their" fence is on the other side of the line, the proverbial fan will get soiled.

My grandparents built a house on the wrong lot; trust me - hire a surveyor.

#7 User is offline   eaparks 

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Posted 25 June 2008 - 07:58 AM

If it is important enough to draw a line in the sand not to cross over, then it must be important enough for it to be accurate, Hire a surveyor. Assuming the terrain isn't to difficult, for a surveyor to establish a straight line, more or less 3,000 feet in length, between 2 known points shouldn't be to expensive.

If you just want the line for a simple indication of where you are in relation to your property line and nothimg more, and don't plan on doing any construction near the line then you could mark the approximate location of your line with something non-permanent like plastic flagging and in 3 years most of it will be gone anyway.

Still I'd use a compass to mark it since it is only 3,000 ft., you will at least get an indication of any error when you sight the other end. Using the GPS if either ends waypoint is off just a little, or what if both happen to be off in opposite directions, then your margin of error could be quite considerable and you have no absolute way of knowing for sure. Recreational GPSs are approximate at best when it comes to repeatable precision, sometimes better than at other times, we just don't know the good times from the bad times with any absolute certainty.

This post has been edited by eaparks: 25 June 2008 - 08:20 AM


#8 User is offline   knowschad 

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Posted 25 June 2008 - 08:21 AM

 JSWilson64, on Jun 25 2008, 06:52 AM, said:

And even if both parties are fine with it, the future owners may not be. When they hire a surveyor and find a fence or other improvements on their property, or find that "their" fence is on the other side of the line, the proverbial fan will get soiled.

My grandparents built a house on the wrong lot; trust me - hire a surveyor.


Very well put. Doing it yourself = bad idea!

#9 User is offline   PastorJon 

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Posted 25 June 2008 - 08:26 AM

Thanks for all the feedback... I've sent this back to the woodlot owner who asked me the question.

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Posted 25 June 2008 - 08:44 AM

 PastorJon, on Jun 25 2008, 09:26 AM, said:

Thanks for all the feedback... I've sent this back to the woodlot owner who asked me the question.

Unless you have Error and Omission Insurance, you shouldn't do this yourself due to the liability. Surveyors have it.

#11 User is offline   PastorJon 

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Posted 25 June 2008 - 08:50 AM

 coggins, on Jun 25 2008, 08:44 AM, said:

 PastorJon, on Jun 25 2008, 09:26 AM, said:

Thanks for all the feedback... I've sent this back to the woodlot owner who asked me the question.

Unless you have Error and Omission Insurance, you shouldn't do this yourself due to the liability. Surveyors have it.


Right... just to be clear... we were not discussing hiring me to do this.... :)

It was more along the lines of the woodlot owners themselves going out with a GPS and marking the line with flags so the loggers would know where NOT to harvest.

#12 User is offline   eaparks 

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Posted 25 June 2008 - 09:07 AM

 PastorJon, on Jun 25 2008, 09:26 AM, said:


Right... just to be clear... we were not discussing hiring me to do this.... :)

It was more along the lines of the woodlot owners themselves going out with a GPS and marking the line with flags so the loggers would know where NOT to harvest.


Since harvesting timber is the reason for wanting to mark the line, definitely have it surveyed. You want to be able to harvest as close to the line as possible to maxime your income, but without cutting the trees right on the line. Wherever the cutting stops, it could possibly give a false sense of where the property line is in the future to others. Most people just assume that when timber is harvested it is harvested up to the line or stops at some other natural obstruction, granted this is a bad assumption but some will still think that.

I don't know what the state laws are for your State but some states set the liabilty damages for cutting timber at 3X the value of the timber cut. So you definitely don't want to infringe on your neighbors property, but still want to maximize your return.

#13 User is offline   julianh 

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Posted 25 June 2008 - 02:47 PM

 eaparks, on Jun 25 2008, 07:58 AM, said:

Still I'd use a compass to mark it since it is only 3,000 ft., you will at least get an indication of any error when you sight the other end. Using the GPS if either ends waypoint is off just a little, or what if both happen to be off in opposite directions, then your margin of error could be quite considerable and you have no absolute way of knowing for sure. Recreational GPSs are approximate at best when it comes to repeatable precision, sometimes better than at other times, we just don't know the good times from the bad times with any absolute certainty.

Echoing the sentiments of others - I totally agree that you should definitely use a surveyor if there is any issue over ownership rights etc. However, if you just want to get a warm fuzzy sense of the overall scope of your "kingdom" (without chopping down any trees near the boundary!) then there is no reason why you shouldn't try marking out the approximate boundaries with a GPSr. If you have a high sensitivity unit, I think you are likely to get better accuracy than trying to sight with a conventional compass through tree cover (unless you have surveyor-quality tools, and the experience to go with it, in which case, the OP's question would not have arisen!) Older GPSrs without high sensitivity chips probably won't maintain a lock under dense tree cover, so may not be up to the job.

A high sensitivity GPSr should be able to retain a 3D lock under most conditions, and should be able to give you repeatable accuracy of better than 10 metres most of the time - 5 metres would not be unusual. If you make a sighting with a compass (assuming you can see 3,000' though the trees!) note that a bearing error of just 1 degree (which would be pretty good accuracy for most non-surveyors!) would give you a lateral error of 16 metres (52 feet) over 3,000 feet.

A key point is that the errors that you get with a GPSr are not accumulative - any single point you mark could be out by 5 to 10 metres, but so is the next one, and the one after that. With conventional surveying techniques of sighting to the next point, then resetting the gear and repeating, the errors potentially accumulate, and closing out a traverse is an important part of any survey - to get a calculation of the total accumulated error, and then average it out over all surveyed points.

This post has been edited by julianh: 25 June 2008 - 02:49 PM


#14 User is offline   PastorJon 

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Posted 25 June 2008 - 05:04 PM

 julianh, on Jun 25 2008, 02:47 PM, said:

 eaparks, on Jun 25 2008, 07:58 AM, said:

Still I'd use a compass to mark it since it is only 3,000 ft., you will at least get an indication of any error when you sight the other end. Using the GPS if either ends waypoint is off just a little, or what if both happen to be off in opposite directions, then your margin of error could be quite considerable and you have no absolute way of knowing for sure. Recreational GPSs are approximate at best when it comes to repeatable precision, sometimes better than at other times, we just don't know the good times from the bad times with any absolute certainty.

Echoing the sentiments of others - I totally agree that you should definitely use a surveyor if there is any issue over ownership rights etc. However, if you just want to get a warm fuzzy sense of the overall scope of your "kingdom" (without chopping down any trees near the boundary!) then there is no reason why you shouldn't try marking out the approximate boundaries with a GPSr. If you have a high sensitivity unit, I think you are likely to get better accuracy than trying to sight with a conventional compass through tree cover (unless you have surveyor-quality tools, and the experience to go with it, in which case, the OP's question would not have arisen!) Older GPSrs without high sensitivity chips probably won't maintain a lock under dense tree cover, so may not be up to the job.

A high sensitivity GPSr should be able to retain a 3D lock under most conditions, and should be able to give you repeatable accuracy of better than 10 metres most of the time - 5 metres would not be unusual. If you make a sighting with a compass (assuming you can see 3,000' though the trees!) note that a bearing error of just 1 degree (which would be pretty good accuracy for most non-surveyors!) would give you a lateral error of 16 metres (52 feet) over 3,000 feet.

A key point is that the errors that you get with a GPSr are not accumulative - any single point you mark could be out by 5 to 10 metres, but so is the next one, and the one after that. With conventional surveying techniques of sighting to the next point, then resetting the gear and repeating, the errors potentially accumulate, and closing out a traverse is an important part of any survey - to get a calculation of the total accumulated error, and then average it out over all surveyed points.


Thank you for that information.

Thanks also to all those who have chimed in! This is greatly appreciated!

At this point, let's suppose that the landowners decide that this is, in fact, the method that they will use to mark a fairly temporary boundary line, that the loggers understand that it's an approximate boundary line, that the land isn't worth anything, and that the owners of the land are pretty easy-going. :)

In other words, let's suppose that the landowners evaluate the cautionary remarks in this thread, and decide that they're not relevant to this particular situation.


Does anyone else want to chime in on the best ways to mark this line? While it is fairly dense woods, it is mostly hardwood, so they will be waiting until fall to mark the line.

#15 User is offline   RRLover 

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Posted 25 June 2008 - 05:37 PM

Some people think "clear-cut" others think "constructive harvest", they're different, depending
on the application. Would a buffer zone, or green belt say 2X the average of the EPEs of 2 or
more handhelds satisfy the land owners, NOT harvesting ALL the timber is always an option.
Don't forget that streams need shade, so harvesting around them is always prudent.

Norm

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Posted 26 June 2008 - 04:51 AM

 PastorJon, on Jun 25 2008, 05:34 AM, said:

How accurate do you believe one could mark a 3000ft long property line in the woods using only a GPS?

Let's say that we knew for sure where the two corners of the property were, and went to get a reading at those two points. Then, we took them back to the computer and used Google Earth (or something) to draw a line... and then take coordinates off the points in the line, and then go back to the woods to draw the line.

Would this work? Would it be accurate within a few feet? If both property owners agreed to do this (instead of hiring a surveyor), would it be fairly suitable?

Are there any tips/tricks anyone would offer on how to do this?


If you want to do it with a handheld gpsr, you might consider purchasing one of these:

http://www.igage.com/thales/

#17 User is offline   Minicup28 

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Posted 26 June 2008 - 07:21 AM

 PastorJon, on Jun 25 2008, 06:26 AM, said:

Would someone evaluate this plan for marking a line:

1) Go to corner A and get the coordinates
2) Go to corner B and get the coordinates
3) Use the GPS to "GoTo" A from B, marking trees with ribbon as we go.
4) Use the GPS to "GoTo" B from A, marking trees with (a different color?) ribbon as we go.
5) Walk from A to B, averaging the ribbon lines (if needed), marking with paint, and removing the ribbons.

How would you improve on this? Alternatively, is there someone who wants to say, "Hey...this is a bad idea... go hire a surveyor"? :ph34r:


I did exactly as you described along a 1500' boundary. The corners were averaged 200x and I repeated the track 3 times dropping flag tipped wires every 20' or so. On the 4th trip we averaged the flag placement by line of sight. I agree it is not legal, nor would I cut wood within 20' of it but it gives my neighbor & I a reference mark to respect. The other 3 boundaries are well surveyed as a result of sales or timber harvesting. By following those marked lines using the same method gave me a good indication of the walking errors that can occur with obstacles and sidehill walking. When it came time to get a building permit it was sufficient to show that I was more than 100' from the line and right-of-way and I saved the $1500 the surveyor wanted to mark the line.

#18 User is offline   PastorJon 

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Posted 26 June 2008 - 07:24 AM

 Minicup28, on Jun 26 2008, 07:21 AM, said:

 PastorJon, on Jun 25 2008, 06:26 AM, said:

Would someone evaluate this plan for marking a line:

1) Go to corner A and get the coordinates
2) Go to corner B and get the coordinates
3) Use the GPS to "GoTo" A from B, marking trees with ribbon as we go.
4) Use the GPS to "GoTo" B from A, marking trees with (a different color?) ribbon as we go.
5) Walk from A to B, averaging the ribbon lines (if needed), marking with paint, and removing the ribbons.

How would you improve on this? Alternatively, is there someone who wants to say, "Hey...this is a bad idea... go hire a surveyor"? :ph34r:


I did exactly as you described along a 1500' boundary. The corners were averaged 200x and I repeated the track 3 times dropping flag tipped wires every 20' or so. On the 4th trip we averaged the flag placement by line of sight. I agree it is not legal, nor would I cut wood within 20' of it but it gives my neighbor & I a reference mark to respect. The other 3 boundaries are well surveyed as a result of sales or timber harvesting. By following those marked lines using the same method gave me a good indication of the walking errors that can occur with obstacles and sidehill walking. When it came time to get a building permit it was sufficient to show that I was more than 100' from the line and right-of-way and I saved the $1500 the surveyor wanted to mark the line.


Thanks for that feedback....

Can you explain averaging to me (or point me to a specific thread that would be helpful to me)? Is this something that most GPSs will do automatically?

#19 User is offline   JSWilson64 

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Posted 26 June 2008 - 10:38 AM

 PastorJon, on Jun 26 2008, 10:24 AM, said:

Can you explain averaging to me (or point me to a specific thread that would be helpful to me)? Is this something that most GPSs will do automatically?

Not all GPS receivers will automatically average coordinates. It's what it sounds like - a running average of the position. You can accomplish the same by marking multiple waypoints at one spot and averaging the latitude and longitude portions. Dump the waypoints using Easy GPS or Mapsource, get those values into Excel, convert to decimal degrees (to eliminate those tricky degree-minute-second conversions!) and average.

The fun thing about doing it yourself is you can visualize every waypoint that went into the average, and the averaged point, in Mapsource or Easy GPS. It'll be pretty obvious if you screwed it up...

I know Garmin 60CSx will do waypoint averaging, can't speak for any others. 76CSx will probably do it, too, since they're basically the same under the skin.

If you're going to do it, I'd let the GPS lock "settle down" for 15 minutes or so before beginning to mark or average. I've done this manually with my Legend to scout a location for a cache (never have gotten around to hiding it, though), and with my 60 CSx, too.

#20 User is offline   PastorJon 

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Posted 26 June 2008 - 12:34 PM

How about time of day?

Should they do an afternoon one day, followed by a morning the next day? Or doesn't it matter?

They're going to wait for fall (it's in a mostly deciduous forest), and I assume they should wait for clear days.

#21 User is offline   reef mapper 

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Posted 26 June 2008 - 12:54 PM

Unfortunately GPS works best at night when the ionosphere has less effect on the signals. For daytime work then late afternoon is the best bet according to some of the articles on the subject. However, this is perhaps more of a concern when using the data for post-processing - I get a noticeable difference in the results between day and night.

Clouds and overcast won't have any affect.

#22 User is offline   julianh 

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Posted 26 June 2008 - 04:11 PM

 reef mapper, on Jun 26 2008, 12:54 PM, said:

Unfortunately GPS works best at night when the ionosphere has less effect on the signals. For daytime work then late afternoon is the best bet according to some of the articles on the subject. However, this is perhaps more of a concern when using the data for post-processing - I get a noticeable difference in the results between day and night.

Clouds and overcast won't have any affect.

True, but if you have a unit with WAAS (and if you can keep a WAAS lock under the tree cover), then the ionosphere errors will be corrected anyway, so this is a bit academic. I think a bigger factor on real-world accuracy is the current distribution of visible satellites in the sky above. At one time, there will be a good spread of satellites from horizon to horizon and overhead, and this will give you the best overall accuracy. An hour or two later, the only visible satellites may be all clustered in one part of the sky, and this will give relatively poor accuracy.

In my experience, there is not much practical difference in accuracy in hand-held GPSrs between day and night specifically, but you can get significant differences in accuracy from hour to hour. For best results, you should repeat the "survey" on two or more runs, several hours apart. It won't help much to repeat at the same time the next day, as the satellites end up in more or less the same locations every 24 hours, so you would really be checking the repeatability of your GPSr under almost identical conditions, rather than getting two independent sets of measurements.

You could use a tool like "Trimble Planning" to get an idea of how the theoretical accuracy of your GPSr at your location might be for any time - it will show you the DOP (indicator of theoretical accuracy), no. of visible satellites, location of the satellites, etc. Surveyors use tools like this to help them know when the optimal time to carry out a GPS-based survey will be. Free download from: http://www.trimble.c...gsoftware.shtml

Hope this helps!

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Posted 26 June 2008 - 06:47 PM

I tend to forget that up there you can access WAAS (now where's the envy smiley). I've had to focus on setting up a base at home for post-processing (Delorme's PostPro, Eartmate and Blue Logger setup) to improve my data, which has meant many hours of logging at different times, hence the observation about day vs night.
But all a bit OT!
Good point about the Trimble planner which is especially useful if you know you'll have obstacles like an adjacent mountain etc.

#24 User is offline   Eaglecacher55 

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Posted 26 June 2008 - 08:08 PM

Since most would agree that the situation is not that you are trying to draw a legal line (although whether you should might be debated), we are talking about opinions....... so I got one too :D I live south of you in Hardin Co and have had this exact situation. Both of us went out, really neither of us had an idea of exactly where the line was, but came to an agreement and the other owner harvested (pretty much by clear cut :D ) and there was no issue, although I guess I could have taken the issue to court had there been a problem because there was nothing at all in writing that I had agreed too. I wish that I would have had the GPS that I have just ordered (60HCx) to do exactly what you are talking about (pretty much because I am a "type A" personality and like to maintain control LOL . This idea will not impact the rights of future owners because there is really no property line being establish other than for the harvesting of timber, so IMO we are talking about a current agreement between owners. They can go out and do this by agreement or they could do it by GPS which would tend to act as a non-biased mediator for both of them. Personally, I would want to type up something that describes what is to be done (agreed upon method etc), state that it is not intended to be a property line for future use, set an acceptable buffer zone (approx accuracy of the GPS or a little more), and have it signed and notarized with both parties receiving a copy. I am almost certain that this would protect each of them in a court of law, although I am sure that any surveyor or any lawyer would tell you differently, primarily because they are in that business and most of the time they want to "give you the business" at a fee. If either have a lawyer friend that would give them advise without the desire to make a client out of them, I am certain that he will confirm this, and might even type up something for them an a very minimal fee (there are actually lawyers that are friends--bet that draws fire) Also personally speaking, I would make temporary markings (stick wires with red flags) that can be taken up after the project and harvesting is complete, so that they do not get mistaken or construed for a "legal" survey in the future. I just don't see the need to pay to have the line set, NOR to pay $2000 for a more accurate GPS, that although it would be much more accurate, won't be the least bit more legal in this situation. If possible I think something in the "high sensitivity" receivers would be great and possibly increase the accuracy to an very acceptable level.

Spoken as a property owner that believes your method to be legal and acceptable if agreed upon by both parties..... not a a lawyer or surveyor, or as a liability paranoid by stander. Might not keep you out of court, but then going "legal" all the way won't always keep you out of court either. I would much rather have a signed and notarized agreement of my counterpart on my side as to pay a surveyor and still risk a disagreement that could be in court.

#25 User is offline   PastorJon 

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Posted 07 July 2008 - 12:44 PM

 julianh, on Jun 25 2008, 06:00 AM, said:

While you could create a string of way-points along the line, a better way to mark out a straight line between two points is to navigate to the first point, then set a course for the second point. Use the off-course indicator (available on all Garmin hand-helds, I think, as well as most other hand-held GPSrs) to tell you how far you are to the left or right of your intended straight line course. That way, you can accurately walk the whole line, with an accuracy limited only by the accuracy that your GPSr can attain under the conditions, not just the finite number of points you have set up.

Hope this helps!


Can someone talk to me about the "Off-course indicator"???? Is this something different than simply using the "GoTo" feature? What models is this available on, and how does it work? Thanks!

#26 User is offline   murja 

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Posted 09 July 2008 - 05:17 AM

As a Land Surveyor I love to read this kind of stuff. You wouldn't believe the work I get "fixing" what laymen have screwed up. There are so many pitfalls, some of which are outlined above, you wouldn't believe it.

#27 User is offline   table/cache 

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Posted 12 July 2008 - 11:05 AM

If you want a legal line, and there is money involved (ie tree harvesting), I'd say the person profiting should be willing to pony up for a surveyor. In fact, I wouldn't agree to anything else.

If somebody is doing this to get a sense of where a property boundary is, then I'd suggest the compass method.

I just spent 9 days doing almost exactly what you describe. We were running 700m long soil sample lines in moderate to heavy brush.

We would average a waypoint for 10 minutes or so - the longer the better. We worked in UTM, and once we had a waypoint averaged, we would look at what the offset was and move to the proper spot. The we would follow the compass bearing to the other waypoint. With practice you can be only a few metres off at the destination. Measuring distace gets complicated, but the actual line is not so hard to follow with a sighting compass. In my mind the compass has the advantage that it point exactly where you want to be going. You just have to make an effort to stay on line, and trust it.

GPSr are great tools, but they aren't always the most appropriate.

#28 User is offline   twolpert 

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Posted 12 July 2008 - 12:05 PM

Don't use a recreational handheld for this purpose. All you have to do is look at your EPE (estimated position error) to tell you it's a bad idea. On a typical handheld recreational unit, EPE will be in the 15 to 20 foot range under normal conditions. This is a probabilistic estimate of the actual error. None of the recreational manufacturers will actually admit to which calculation they use, but typically the confidence level will be in the 50% to 68% range. In practical terms, this means that there is a 50% (or 68%, or whatever) probability that the real position is within a circle of EPE radius around the reported position. And a 50% (or 32%, or whatever) probability that the error is larger than that.

If your unit actually uses the 50% CEP (circular error probability) measure, you would have to multiply the EPE by 2 to get a 95% confidence level. So if your unit reports an EPE of 18 feet, you have a 50-50 chance of the actual position being with 18 feet of the reported position (and the error could be in any direction). There is a 95% probability that the actual position is within 36 feet of the reported position (with error in any direction).

For your application, 72 feet is a lot of trees. Surveryors use commercial GPS units with sub-centimeter accuracy. But these things cost 10s of thousands of dollars. A recreational handheld is no substitute.

Edited to add: Position averaging helps. So does WAAS. But neither will actually solve the problem. The typical recreational EPE is just too high to overcome.

This post has been edited by twolpert: 12 July 2008 - 12:08 PM


#29 User is offline   geode hunter 

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Posted 13 July 2008 - 06:16 AM

 PastorJon, on Jun 25 2008, 10:26 AM, said:

Would someone evaluate this plan for marking a line:

1) Go to corner A and get the coordinates
2) Go to corner B and get the coordinates
3) Use the GPS to "GoTo" A from B, marking trees with ribbon as we go.
4) Use the GPS to "GoTo" B from A, marking trees with (a different color?) ribbon as we go.
5) Walk from A to B, averaging the ribbon lines (if needed), marking with paint, and removing the ribbons.

How would you improve on this? Alternatively, is there someone who wants to say, "Hey...this is a bad idea... go hire a surveyor"? :unsure:


obviously the recreational gpsr, that we have all come to love, will not get you a line that would stand up in court.

If all of my survey equipment was broke and I could not find someone to loan me one, this is how i might try it , for fun, with my gpsr.

Mark a point, on your gprsr, at each property corner.
starting at one corner. fasten an end of a high tensile electric fence (not the cheap stuff. you are going to stretch this really tight)

Start unrolling the fence towards the second point using the directional compass on the gpsr to guide you thru the woods. look back once in a while to make sure your line is straight.

Once you have reached the property corner, use the tensioner that is is recommended for the high tensile fence wire and get it really tight. walk the line several times picking it up and removing the brush that might be keeping from being straight. You might have to back track the line to get on the correct side of the trees you want to keep. Add more tension to the wire after each trip.

This will take some time. After several trips back and forth you should have a fairly straight line.

4000 feet of fence wire is about $120 ?
The cam type tensioners $10 ?
Tensioner tool $20 ?

A Topcon GMS-2 Handheld GPSr boosts 1cm accuracy,but costs $5000+ or
Sokia total station and data collecter $8000+- or
Professional surveyor to stake out your line $800+-

Peace of mind at a court appearance - Priceless -

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