data accuracy calculation & truth checking

[davideps]

Hello,


I am trying to check the accuracy of some maps I've created in ARCGIS by comparing them to field GPS measurements. I am using a MIO Digiwalker P550 hardware and IGO software. I imported my waypoints (POIs) into ArcGIS using IGO Waypoint Explorer Beta, exporting as a CSV list and importing x,y coordinates. My ARCGIS maps are in a projected reference system (Israel TM grid).

There is a discrepancy of between 50 and 125 meters. Could this be some sort of import (reference system) problem? I am not sure which data to trust more. I am open to comments and suggestions.


thank you in advance,
-david

[GerryC]

Welcome to the forum.

The GPS system uses WGS84 which is based on the globe as a whole. A lot of countries have localised systems (probably much older) that are more suited to the local area. Therefore, they are probably both "correct" but different "correct"s.

I would suspect the Israel TM system is similar to the UK's OSGB system as both are long and thin therefore a projected reference is suitable. As WGS84 is global, no projections are used though. This would explain the difference between the two.

It's best to work with one system though I don't know of any converters between them - if you find one, use it. Either check to see whether your hardware/software has a converter to the Israel TM system or source a map that's already in WGS84 would be my best advice.
_________________
Gerry
TomTom730T
Cameralert for Android
Brodit ProClip mount

[davideps]

Gerry,

Thank you for your response. Two more quick questions, if you have a moment:

1) Are all GPS systems WGS84?
2) Can't I just change the datum using ArcGIS? Mathematically such a conversion is possible, correct?

-david

[GerryC]

WGS84 is the datum of the data sent by the GPS satellites.

As long as the Mio is capable of showing ArcGIS datum co-ordinates, that should be OK. As you say, it's doing the conversions internally. I have no idea what it can do though.

Alternatively, you could do the conversions separately. This site (Google search) appears to have a program that converts to "Israel TM" though I'm sure there's others.

You should not have a problem as long as both the GPS receiving device and the map are using the same datum.
_________________
Gerry
TomTom730T
Cameralert for Android
Brodit ProClip mount

[philpugh]

[davideps]

Thank you Gerry and Phil. I'll play with the conversion a little and see how it goes.

Do either of you (or anyone else) know a good online reference for truth/ground checking with GPS?

-david

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