ArcGIS WGS84 Web Mercator Vs. WGS84 UTM zone 15N: Very different distance dimensions

[CharlyParra]

Hi all, I am working in ArgGIS with a field (raster layer in tif format) in which I have placed two points (.shp layer).

I have two versions of both layers, resulting four layers: one tif+shp set in CRS WGS84 Web Mercator and the other tif+shp set in CRS WGS84 UTM zone 15N.

If I establish the CRS of the Data Frame to WGS84 Web Mercator, the distance between the two points is 138 meters, but if I establish the CRS of the Data Frame to WGS84 UTM zone 15N the distance is 99 meters, which is a huge difference. However, the point layers are displayed absolutely one above the other, I mean: the two points in one CRS haven’t any displacement in relation to the other two points; and the same for the rasters.

1. Why don’t I see both pairs of points misplaced? Is that “on the fly” projection by ArcGIS? If it’s so, does it mean that ArcGIS projects “on the fly” even between data with CRSs of different geographic coordinate system? (as far as I am concerned, ArcGIS only projects “on the fly” between data with CRSs with the same geographic coordinate system but different projection). NOTE: Both projected CRSs (WGS84 Web Mercator & WGS84 UTM zone 15N) correspond to these two different geographic coordinate systems: in ArcGIS, Geographic Coordinate System > Spheroid-based > WGS 1984 Major Auxiliary Sphere & Geographic Coordinate System > World > WGS 1984, respectively.

2. Another major doubt is which of the two CRSs is then better for working in the south of Minnesota State (US).

Thank you for any help or idea given.

[Anonymous User]

Does the Web App Builder widget default to returning geodesic area and distance?

If so, why would they need to explicitly set it as true in Measure distances | ArcGIS API for JavaScript 3.23  

It does appear to be the case that it defaults to Geodesic - Measurement widget—Web AppBuilder for ArcGIS | ArcGIS 

It specifically states that it 'buffers' geodesically (unlike NearMe).  So mostly I just want to confirm with you Melita the Measurement Guru, it's really geodesic by default.   Thanks!

[MelitaKennedy]

Yes, geodesic by default. That actually makes sense because of the wide use of Web Mercator. You don't want to use that. NearMe, because it *should* be used for very local calculations can get away with using whatever coordinate system is being used.

[CharlyParra]

Thanks, Tate Arnold and Melita.

Taking into account what Arnold says and illustrates (“area and distance measurements [in Mercator projections] will not be correct since the farther north you take the measurement, the more distortion there is”), I guess UTM will have similar drawbacks (though surely less limited or less distorted measurements) than Web Mercator projection. Thus, is there a better projection than a Mercator one for area measurement in that latitude (Minnesota)? Or would directly working with geographic coordinate system (WGS 84) be a better solution?

Anyway I need my ArcGIS shape is compatible with another GIS software. According to the information I have, this other GIS software only allows external shapes in “WGS 84 LAT/LON projection”. What can “WGS 84 LAT/LON projection” mean? Is Lat/Lon projection a type of projection? If so, which are the projections within this type? Would UTM projection, or other proposed projection according to the preceding paragraph, be among them?

[MelitaKennedy]

UTM uses a transverse cylinder--the ends are left and right, rather than Mercator's 'upright' cylinder--so it models the fact that the longitude lines converge at the poles better. It's also designed for small(ish) areas, only 6 degrees wide. At the center of the zone, the distortion due to the projection is 1 part in 2500, aka 1 foot in 2500 feet. You could also use a Minnesota State Plane zone for the area. Its distortion is more like 1 part in 10000. Minnesota also has county-level coordinate systems which will have less distortion.

WGS 83 LAT/LON means that they want latitude/longitude values aka the data in a geographic coordinate system.

Melita

[CharlyParra]

WGS 83 LAT/LON means that they want latitude/longitude values aka the data in a geographic coordinate system.

Why the word "projection": “WGS 84 LAT/LON projection”? The inclusion of the word "projection" is confusing, do you see it clearly anyway?

And, is a geographic coordinate system (such as WGS84) a good choice for area measuring within ArcGIS? Or, on the contrary, must we always choose the proper projected coordinate system for that purpose as the most suitable option?

Thank you.

[MelitaKennedy]

They're conflating the term 'projection' with 'coordinate [reference] system'. It may even be from ArcInfo Workstation which did call everything a projection, even if it was lat/lon values.

We're just now starting to incorporate calculating geodetic areas (using the ellipsoid surface), so depending on the version, using a projected coordinate system, particular one that is based on an equal area projection like Albers, will give the best results.