Inaccuracy of magic maps

[+Dewi o Dre-Gwyr]

Has anybody else had the problems with magic maps that I am having,below is an e-mail I sent to our local reviewer.

 

When I type in the bearings I know that the blue location dot is totaly wrong,this then gives the reviewer a false location which leads to the problems below.

 

Hi there, With reference to the caches I am trying to have published in Penllergaer Woods and your review of them.

All the caches I am trying to have published are definitely more than 165m apart. I have re-checked them several times using an e trex 30 and an oregon 650t and both gps devices give the distances at more than the required 165m in all directions. I know from walking the distances several times that there is more than 6m between caches in any direction, even as the crow flies.

I remember being advised to use 'Magic Maps' to measure distances by Geocaching.com and I found them to be very inaccurate. If it is Magic Maps that reviewers use to check co-ordinates, to prove a point, we have taken 3 different trig points i.e. Mumbles Hill ss624-875 and found it to be 72m out on Magic Maps, Mynydd Dinas ss760-915 found to be 89m out and finally Caldey Island ss142-959 which was 67m out. I sourced the Trig point bearings from Trigpointing.uk. Even my house is in the wrong street on magic maps.

If it is not Magic Maps that is used to review geocache placements could you please advise me which maps are used. I have gone to great effort not to waste your time and I cannot understand how our measurements could be so different. Thank you for your time.

[Keystone]

Reviewers everywhere use the Geocaching.com database and site algorithms to measure distances between physical cache elements. For a new cache that's too close to a previously published traditional cache, when you click on the "Find all nearby caches" link on the new cache submission, you would see that the distance shown in the search results list matches the distance cited by the reviewer in their note to the cache setter.

 

The magic maps are fabulous tools for many purposes, but measuring distance isn't one of them.

Edited December 10, 2014 by Keystone

[+Gary & Jane]

The T:UK page for Mumbles Hill is http://trigpointing.uk/trig/4982 which shows an OS grid reference of SS 62408 87568. This grid reference should be accurate to the nearest 1m. A grid reference of SS 624 875 is only accurate to 100m. If you give a 6 digit grid reference to MagicMapIt! then it will use a 10 digit grid reference of SS 62450 87550 as the position to display the arrow.

 

Passing the full 10 digit grid reference to MagicMapIt! gives the following link http://benchmarks.or...?c=SS6240887568 . You should see the arrow pointing very close (within 2m) to the centre of the trig point.

 

Similarly taking the full 10 digit grid references from the T:UK pages http://trigpointing.uk/trig/5013 and http://trigpointing.uk/trig/1873 gives MagicMapIt! links of http://benchmarks.or...?c=SS7607591541 and http://benchmarks.or...?c=SS1424495949 . In both cases the arrow is pointing very close to the centre of the trig points.

 

I've tried using the measuring tool on the Magic map and it does appear to become more and more inaccurate the further west or east you go from the 2 degrees west longitude line.

 

I would therefore say that Magic maps are accurate for positioning but can't be relied on for distance measurements in certain locations. Hope this helps.

[jri]

Another potential source of errors is the difference between Ordnance Survey (OSGB 36 or National Grid) and GPS (WGS84) coordinates. MAGIC uses Ordnance Survey, while Geocaching.com uses WGS84, and same lat/lon coordinates can refer to points over 100m away from each other, depending on which system you are using. To get an accurate distance between two points, you need to make sure that the measurements from each location have used the same datum. See http://en.wikipedia....B_36_and_WGS_84 for more details.

 

The Geocaching Map Enhancements userscript has a feature that lets you measure distances between points on the Geocaching.com map page, which you may find helpful (it can also be used to measure out routes). There is more information about GME at http://geo.inge.org.uk/gme.htm

[+Dewi o Dre-Gwyr]

Thanks for the replies so far.