Waypoint - error in positioning ?

peter3887 · April 15, 2015, 05:47:42 PM

Hello Orux people - first, many thanks for a great app (I even made a small donation :-)

I am learning how to create maps to load for offline use, and have successfully calibrated and loaded a number of maps so far using the Orux desktop program. I though that I could check the accuracy of the calibration by creating some waypoints with known latitude/longtitude (determined from Google & Bing maps) then displaying them on the maps to check that they appear in the correct place. Unfortunately they seem to be out by several 100 feet. I first assumed that I had made errors in the map calibration, but when I visited the location in person, the GPS track is spot on. Checking further, I have matched the Latitude/Longtitude from (1) Google & Bing maps, (2) the actual GPS figures recorded in the track GPX file and (3) the properties of the waypoint. All three are the same to the 4th / 5th decimal place which I think is 10s of feet. The maps (1) and the GPS (2) line up, but the waypoint (3) displays 500ft South West. Am I missing something obvious, or is there a problem with the display of the waypoint ?

Thank you for any advice

Peter

orux · April 17, 2015, 03:08:09 PM

~~Quote from: "peter3887"~~Hello Orux people - first, many thanks for a great app (I even made a small donation :-)

I am learning how to create maps to load for offline use, and have successfully calibrated and loaded a number of maps so far using the Orux desktop program. I though that I could check the accuracy of the calibration by creating some waypoints with known latitude/longtitude (determined from Google & Bing maps) then displaying them on the maps to check that they appear in the correct place. Unfortunately they seem to be out by several 100 feet. I first assumed that I had made errors in the map calibration, but when I visited the location in person, the GPS track is spot on. Checking further, I have matched the Latitude/Longtitude from (1) Google & Bing maps, (2) the actual GPS figures recorded in the track GPX file and (3) the properties of the waypoint. All three are the same to the 4th / 5th decimal place which I think is 10s of feet. The maps (1) and the GPS (2) line up, but the waypoint (3) displays 500ft South West. Am I missing something obvious, or is there a problem with the display of the waypoint ?

Thank you for any advice

Peter

Hi, Peter, thanks;

Sure it is a map calibration problem.

To test the map calibration:

1.-open the map, center the map to a well know position (a road cross, a build,...)

2.-load an online map; the map must be loaded exactly over the same point.

Or:

open the map, start the GPS, check that your position is right.

If the map is right, may be it is a problem with the intrudution of the coordinates; give us an example.

orux

peter3887 · April 27, 2015, 11:07:49 PM

Hi Orux - Thanks for your reply.

I have checked my map calibration carefully, and it is not in error enough to explain the error in the display of the waypoint. When I visit the place on the map in person, the GPS and the track recorded and displayed on the map are very close to the actual position.

I made some further investigation, and looked at a copy of the oruxmaptracks.db using the SQLite Manager extension for Firefox. In the pois table I see an entry for the Waypoint that is causing me problems, but something is strange. In the database I see Lat/Long 50.9162036, -1.1679015, but when I look at the properties of the waypoint using the program on the phone (Waypoints, Manage, Edit Properties, Coordinates) I see Lat/Long 50.91714, -1.165991 (These are the coordinates I entered manually).

I have entered the coordinates that I see in the database into Bing Maps, and the location shown is very close to where the waypoint is displayed by the program on the phone. So this confirms that the map calibration is not too far out, and the program is correctly displaying the coordinates from the database.

Is it possible that, when I entered the coordinates manually into the phone, I somehow did not 'confirm' the change so it did not update the database ? Or is there some calculation / transformation carried out between the data I enter and the data recorded in the database ? Can you think why the coordinates displayed on the phone are different from the figures in the database ?

Please do not think that I am being too critical - I am very pleased with the program and thank you again for your work on it. The problem with the waypoint is more of an annoyance - it does not stop the program working very well overall, and it is just something that I do not understand. I bring it to your attention in case there is a bug somewhere that could be easily fixed.

Best Regards

Peter

orux · April 28, 2015, 11:02:51 AM

~~Quote from: "peter3887"~~Hi Orux - Thanks for your reply.

I have checked my map calibration carefully, and it is not in error enough to explain the error in the display of the waypoint. When I visit the place on the map in person, the GPS and the track recorded and displayed on the map are very close to the actual position.

I made some further investigation, and looked at a copy of the oruxmaptracks.db using the SQLite Manager extension for Firefox. In the pois table I see an entry for the Waypoint that is causing me problems, but something is strange. In the database I see Lat/Long 50.9162036, -1.1679015, but when I look at the properties of the waypoint using the program on the phone (Waypoints, Manage, Edit Properties, Coordinates) I see Lat/Long 50.91714, -1.165991 (These are the coordinates I entered manually).

I have entered the coordinates that I see in the database into Bing Maps, and the location shown is very close to where the waypoint is displayed by the program on the phone. So this confirms that the map calibration is not too far out, and the program is correctly displaying the coordinates from the database.

Is it possible that, when I entered the coordinates manually into the phone, I somehow did not 'confirm' the change so it did not update the database ? Or is there some calculation / transformation carried out between the data I enter and the data recorded in the database ? Can you think why the coordinates displayed on the phone are different from the figures in the database ?

Please do not think that I am being too critical - I am very pleased with the program and thank you again for your work on it. The problem with the waypoint is more of an annoyance - it does not stop the program working very well overall, and it is just something that I do not understand. I bring it to your attention in case there is a bug somewhere that could be easily fixed.

Best Regards

Peter

Hi, Peter;

have you changed the map datum to another different than wgs84?

If you are using a custom datum, the wpt creation form, and all the other forms, the coordinates are transformed to that coordinates system.

You can see your current datum if you set the control in configuration--user interface--dashboard--user interface--map datum.

You can change the datum under maps button in te map viewer.

orux

peter3887 · April 29, 2015, 05:22:44 PM

Hi Orux - Thanks again for your reply. Hopefully these forum notes will also help other users

Your suggestion has solved the problem - When I displayed the Datum as you suggested it was set to European something - I could not see which one as it only displays the first characters and there seem to be a number of different European datums.

I don't know how this happened - I have tried using the Map Calibrator previously , but I think that only affects the map currently displayed and any changes are not saved ? is this correct ? How is the original default datum set as I don't remember the app asking for any datum when I first used it ?

I changed the datum to WGS 1984 as you described - note that the map I am using was calibrated with your desktop application using this WGS1984 datum and the 'flat earth' projection. I then deleted the waypoint and created a new one with the same Lat/Long coordinates - it is now displayed on the map in exactly the correct position :-) :-).

This does raise a related question. I have been recording tracks on some of my walks and I note that some display with a systemic offset, ie the whole track is shifted East by 200ft. I had assumed that this was a map calibration problem which was why I was experimenting with setting Waypoints to try and check the maps. Exporting the tracks from the phone as GPX files and importing to Bing maps shows the tracks in exactly the correct position, so the GPS is obtaining the Lat/Long accurately. And now that the waypoint displays correctly I am thinking that my map calibration is not as far out as I feared. Is there a similar transformation of coordinates when displaying the tracks ? If so, can I assume that new tracks recorded with the Datum correctly set will display more accurately ?

Thanks again for your help

Peter

orux · April 29, 2015, 05:37:25 PM

~~Quote from: "peter3887"~~Hi Orux - Thanks again for your reply. Hopefully these forum notes will also help other users

Your suggestion has solved the problem - When I displayed the Datum as you suggested it was set to European something - I could not see which one as it only displays the first characters and there seem to be a number of different European datums.

I don't know how this happened - I have tried using the Map Calibrator previously , but I think that only affects the map currently displayed and any changes are not saved ? is this correct ? How is the original default datum set as I don't remember the app asking for any datum when I first used it ?

I changed the datum to WGS 1984 as you described - note that the map I am using was calibrated with your desktop application using this WGS1984 datum and the 'flat earth' projection. I then deleted the waypoint and created a new one with the same Lat/Long coordinates - it is now displayed on the map in exactly the correct position :-) :-).

This does raise a related question. I have been recording tracks on some of my walks and I note that some display with a systemic offset, ie the whole track is shifted East by 200ft. I had assumed that this was a map calibration problem which was why I was experimenting with setting Waypoints to try and check the maps. Exporting the tracks from the phone as GPX files and importing to Bing maps shows the tracks in exactly the correct position, so the GPS is obtaining the Lat/Long accurately. And now that the waypoint displays correctly I am thinking that my map calibration is not as far out as I feared. Is there a similar transformation of coordinates when displaying the tracks ? If so, can I assume that new tracks recorded with the Datum correctly set will display more accurately ?

Thanks again for your help

Peter

Hello, Peter;

The application always uses the WGS84 datum, unless the user explicitly changed it. You might not have noticed, but the app never changes the working datum itself.

Your map is wrong calibrated. 'flat earth' is not a projection. It only serves to when the map has an unknown projection. You should use the projection with which your map was done.

All data is stored in the databases in WGS84 system. If the datum is changed only affects how coordinates are displayed on the screens.

orux

peter3887 · May 03, 2015, 01:28:55 PM

Hi Orux - Thanks again for your reply. I have recreated my maps using your desktop application, and Datum = WGS84 Projection = Latitude/Longtitude instead of Flat Earth (these are for bits of Ordnance Survey 1:25000 maps). The accuracy is now very much better - so I made this reply to help anyone else who may be looking at the forum.

Regards

Peter