etrex 10 display problem

[Wobbly-1]

Has anyone else noticed that if you save a geocache in OS grid co-ords using garmin communicator from the web page the OS position on the etrex geocache page is not the same as shown on the web page. This would only appear to be a display problem as if you then save the geocache on the etrex to a waypoint the position is displayed correctly. I have been in touch with Garmin support and they recommended a full reset and software update but this does not resolve the problem and they have not offered any other explanation for the error.

 

Any explanation would be gratefully received (or is it just me).

Edited May 22, 2013 by Wobbly-1

[Wobbly-1]

When you say the 'position is different' what do you mean? Is the grid ref itself different? Or does it simply not look in the correct position on the map? If the latter bear in mind the map that comes with the Etrex 10 is neither accurate or detailed.

 

Nothing to do with the map. This only seems to happen if you are working OS grid, if you go to the geocaching data page on the GPS the co-ords are not the same as those shown on the web page on the gc site before downloading. I think it is just a display algorithm problem on the page because the positional data is correct if you save it in wgs 84, it is also correct if you save the geocache on the gps to a waypoint.

[+The Patrician]

Could the wrong datum be in use?

 

Should be OSGB for OS refs.

WGS84 for lat & long.

 

The difference is about 120 metres from memory.

 

Edit from Wikipedia:

"Datum shift between OSGB 36 and WGS 84

 

The difference between the coordinates on different datums varies from place to place. The longitude and latitude positions on OSGB 36 are the same as for WGS 84 at a point in the Atlantic Ocean well to the west of Great Britain. In Cornwall, the WGS 84 longitude lines are about 70 metres east of their OSGB 36 equivalents, this value rising gradually to about 120 m east on the east coast of East Anglia. The WGS 84 latitude lines are about 70 m south of the OSGB 36 lines in South Cornwall, the difference diminishing to zero in the Scottish Borders, and then increasing to about 50 m north on the north coast of Scotland. (If the lines are further east, then the longitude value of any given point is further west. Similarly, if the lines are further south, the values will give the point a more northerly latitude.) The smallest datum shift is on the west coast of Scotland and the greatest in Kent."

Edited May 22, 2013 by The Patrician

[Wobbly-1]

Could the wrong datum be in use?

 

Should be OSGB for OS refs.

WGS84 for lat & long.

 

The difference is about 120 metres from memory.

 

Edit from Wikipedia:

"Datum shift between OSGB 36 and WGS 84

 

The difference between the coordinates on different datums varies from place to place. The longitude and latitude positions on OSGB 36 are the same as for WGS 84 at a point in the Atlantic Ocean well to the west of Great Britain. In Cornwall, the WGS 84 longitude lines are about 70 metres east of their OSGB 36 equivalents, this value rising gradually to about 120 m east on the east coast of East Anglia. The WGS 84 latitude lines are about 70 m south of the OSGB 36 lines in South Cornwall, the difference diminishing to zero in the Scottish Borders, and then increasing to about 50 m north on the north coast of Scotland. (If the lines are further east, then the longitude value of any given point is further west. Similarly, if the lines are further south, the values will give the point a more northerly latitude.) The smallest datum shift is on the west coast of Scotland and the greatest in Kent."

 

That certainly could be the explanation and would explain why the offset varies slightly from Geocache to Geocache. As I recall the OS grid is presented in squares whereas WGS84 is strictly in a slightly curved trapezoid shape and to convert to OS grid there was what was referred to as a "rubber conversion", to get the OS grid to convert between the two. If that is the explanation then Garmin, as I thought, have used a different algorithm for the Geocache display page on the GPS than for the Wapoint page which although strange could explain everything. Just a pity that they don't appear to be interested!

[Wobbly-1]

Well I have an Etrex 20 that is permanently set to OS grid, and caches are spot on so I don't believe it's a Garmin problem. Does it show Britsh Grid, Ord Srvy GB and Airy when you go to Set Up, Position Format?

 

Incidentally it's been posted before that the grids/locations on the geocaching.com site are deliberately 'offset' to ensure you download them rather than simply copy the grid ref. Why that might be so is beyond me

 

Yes that is exactly what is shown, if you select British Grid the other settings are fixed. Its interesting that your Etrex 20 does not demonstrate the same positional difference. I have never heard of the GC page being deliberately offset I have often in the past just copied the data by hand and never had any problems. As previously stated if the Geocache data on the GPS (which has been downloaded using Garmin Communicator) is then transferred to a Waypoint on the Etrex the positional data is as shown on the GC webpage it is only when you display the individual Geocache under Geocaches, on the Etrex, that the difference becomes apparent.

Edited May 22, 2013 by Wobbly-1

[Wobbly-1]

I have resorted to taking a picture of the etrex geocache display page which shows that for Cache GC3D4AA the displayed information is NH 44162 58250 whereas the correct datuum is NH 44243 58537, unfortunately there does not appear to be a way of attaching the image. This offset is much the same for all geocaches downloaded. It does not affect the accuracy of the gps but is still a puzzle as to why it happens.