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Field Maps shows conflicting Lat/Long values on same point

MarieDawes

Hello all,

My organization recently purchased an external receiver for field data collection. I've been experimenting with a workflow for updating our existing points with more accurate gps data. I set up Field Maps based on a map of existing points. I then updated some of those points with our new gps receiver in the field. Now when I look at those points in Field Maps I see two different Lat/Long coordinates, shown below. Has anyone else seen this happen, and why are the values different? (I believe the values shown in the table are correct, as per our GPS unit; I don't know where the values in the sub-header are derived from).

Thanks for the help.

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Accepted answers

ColinLawrence

Any transformation will introduce a margin of error, there is no getting around that and it does not mean you have done something wrong. The coordinates in the header are not coming from the iPads internal GPS as there would be a much larger difference in the coordinates than what is in your screenshot. They are coming from your Trimble receiver, but are being projected onto your map which uses the transformation defined in the Location Profile, and the resulting location is stored in the feature Geometry and shown in the popup header. The value in the gnss metadata are the raw geographic location (unprojected). Does that help explain it better?

All comments

MarieDawes

Thank you! Now I understand. This is exactly the explanation I was looking for.

ColinLawrence

MarieDawes

Thanks @ColinLawrence , @erKay_nv5 and @RobertBorchert . I spent quite a lot of time figuring out the setup and specifying the transformation. I think I have that right, as I checked a point I collected against its known coordinates from NGS and it was very, very close.

The part I wasn't understanding is where the coordinates in the popup header were coming from. If they're pulled from the mobile device - in my case an iPad - that makes some sense.

RobertBorchert

The x,y you see on your Field Maps application is based on your mobile device and x,y you took is based on your collection. your mobile device such as a phone are tablet can be off by quite a few feet

erKay_nv5

This is not that inaccurate for field collection on a known data point. Very few points are going to match up perfectly to a listed coordinate, just due to the variations of real-time GPS satellite positions, especially past the third or fourth decimal. It also depends on your external receiver and how you have positional correction set up.

ColinLawrence

Hi @MarieDawes , the coordinates in the GPS metadata (Lat, long fields in your screenshot) are raw values from the receiver and as you note these match what is shown on your GPS receiver or receiver app. The values in the popup header come from the features geometry which could be slightly different depending on the basemap being used, Location Profile set up and the transformation(s) used. I hope this helps!

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