Is it a good idea to use AGOL basemap with GPS in Collector?

[simoxu]

Hi there,

I am thinking of using Collector to collect data in the field, but I am not sure what base map I should use.

ESRI provides lots of base maps with great cartography, but my concern is the projection. as you know, those ESRI base maps  (maybe not all of them) are using Web Mercato Auxiliary Sphere (WKID: 102100)  which used to be a very controversial projection (Google/Bing maps started using it) and is not very accurate. although it's well accepted by the GIS industry now, I am wondering whether or not it will cause problems when using GPS in the Collector?

I don't know what kind of GPS  my users will have on their devices, but we know all the GPS will use WGS84 datum. Web Mercato Auxiliary Sphere is using the same datum, will this guarantee there won't be any accuracy loss when displaying the GPS locations on the map? we don't really care about the accuracy to the true location, only relative accuracy to the map matters.

I did some test using my phone, the result is not too bad, but don't know if that's just a individual case, or it's guaranteed to be working on other devices as well.

I am in Australia, the most popular projections in my work are GDA1994_MGA (283xx) and GDA1994 Lambert (3017)

Thanks,

Simo

[RonKnepper]

I'm in the us and I really haven't seen this as an issue on many tests I've done.  There is some offset at times but really the basemaps are used for reference.  I wonder why the accuracy of the basemap is a concern though?  I'd assume as this is for field data collection you would be more concerned with the accuracy of the gps device being used and you can set accuracy settings within collector.  The field people will be standing in the collection locations so no matter where it shows on your basemap your data would be based on the device gps.  Now you can select a point in collector that is not the physical location but we only ever do this in testing or backoff (and when accuracy is not needed to that level).  If your teams are not able to stand in the exact locations and the accuracy is that much of a concern maybe look into range finders that they can set gps from a distance.

[simoxu]

Hi Ron,

Thanks for your reply. let me clarify my situation, I am NOT going to collect the information of the GPS location, instead, I am using GPS to "identify" the properties and assess the properties. so knowing where I am on the map is quite important and the more accurate the better

I reason I raise the concern was that I had a thought on GPS maps then I did a bit Google research on it. I can assume the GPS receiver is always giving me the right coordinates (lat/longs or other units in UTM projection). but the 2D map I am using to show the GPS dot is a result of particular projection, which is based on a datum and a projection method. by this point, maybe the question should really be: is the GPS and the map I am using are using the same datum? because I think,  if the datum is the same, the actual lat/long in two systems (GPS and the map) for a location on the earth is the same, they should match.

in definition for WGS84 and Web Mercato Auxiliary Sphere, it seems the datum is the same, so maybe it should not be a question... if I am using  another Geographical Coordinate System in my map, there may be a problem, the GPS location on the map can drift away (for example, I am standing in a property, but the GPS shows I am standing out of the property on the map)

I am curious what map projection do they use in the commercial GPS Unit in our cars.

Hopefully I am not making an obvious thing too complicated. I am not a surveyor, they should know these things better.

[MiaogengZhang · Accepted solution]

This should not be an issue.  Here is an article related to this topic: What does the NGA “Web Mercator” advisory mean for Esri Defense and Intelligence users? | ArcGIS Blo... .

Regarding using another Geographical Coordinate System in your map, the GPS location on the map can drift away. What device do you use? What is GPS accuracy?

[simoxu]

Hi Miaogeng,

Thanks for referring me to the blog. it sounds like if there are no datum transformation involved, assuming the GPS accuracy is acceptable, then the GPS position and the map should match.

Miaogeng Zhang​ Gordon Sumerling​

I don't know what device the users will have at the end, because they are from different government agencies, I don't think they will have uniform devices. maybe some use android phone, some use iphone&ipad, windows phone when collector is available on that platform. I hope we don't need external GPS units, but I'll see after the first field test.

Simo

[GordonSumerling]

Simo,

The variance you refer to between map projection of the basemap and the GPS on the device you are using going to be relatively minor. if location is important then you should be more concerned with the accuracy of the GPS receiver. Accuracy of location is dependant not just on the device but also the surroundings, such as collecting under a tree or near a building. the surroundings influence on accuracy can very from +/- 3m to anywhere over +/- 10m.  The you have the number of available satellites. Finally if you do require higher accuracy they you should ensure that GPS signal is being corrected. In the case of collector this is usually via Licia Zeno or Trimble R1.

Just some thoughts.

Gordon