topic Re: ArcGIS WGS84 Web Mercator Vs. WGS84 UTM zone 15N: Very different distance dimensions in Coordinate Reference Systems Questions

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

CharlyParra — Fri, 15 Aug 2014 13:44:27 GMT

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.

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.
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.

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

TateM — Thu, 04 Sep 2014 02:58:09 GMT

Charly, I sometimes experience the same thing. What I think going on is that when using Web Mercator projection it gets really distorted closer to the poles (north, and south), and its also projecting the entire planet. With the UTM Zone, things will be very accurate or it supposed to be accurate for that particular zone, in your case UTM Zone 15N. I think it projecting particular zone vs. entire planet. So I would use UTM Zone for a better accurate measurement.

I also noticed that in ArcMap, not only distance are off, but area as well (square mile, acres, etcs.). Its different when measuring for area using WGS84 Web Mercator vs. UTM zone or StatePlane. I'm guessing ArcMap doesn't read geodesic spherical well?

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

ArnoldKuilman — Thu, 04 Sep 2014 06:32:55 GMT

Like any projection, Web Mercator has inherent limitations. First, you will notice that no data is cached at the North or South Pole. This is a limitation of all Mercator projections. Also, area and distance measurements will not be correct since the farther north you take the measurement, the more distortion there is.

An example is in the picture below, a tissot indicatrix, showing the distortion in projected area.

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

MelitaKennedy — Thu, 04 Sep 2014 20:00:43 GMT

To expand on Arnold's reply, Web Mercator is designed for viewing and ease in creating/storing/retrieving cached imagery tiles. It's not designed for calculating distances or areas (if you get correct answers, then the software is calculating in something else!).

The fact that the points all lie on top of each other is due to the project on-the-fly capabilities of ArcMap. Whichever coordinate system (CS) the data frame (map) is using, any layers in a different coordinate system are being converted in the data frame's CS. (1)

You have a weird case, in that WGS 1984 Major Auxiliary Sphere isn't a "real" geodetic GCS (datum). Coordinates are the same as in WGS 1984, but are treated as if they're on a sphere with radius 6378137.0 m. If there were two or more geodetic datums involved, you would need to pick transformations. However, ArcMap will still project on-the-fly even if you don't set any transformations. Basically, it just skips the transformation step.

Notice how the longitude lines are parallel in Web Mercator. As a rough guide, that means that the true east-west distance is about cos(latitude) of what's reported. This fits well with what you're seeing: 99/138 = 0.717, cos(44) = 0.719.

You're much better off using the UTM zone, or possibly switching to the SPCS zone (although that's not on WGS84).

Melita

(1) If you want to see what the data looks like with project on-the-fly turned off, click the globe on the Coordinate Systems tab of data frame properties and select the Clear option.

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

TateM — Fri, 05 Sep 2014 12:46:24 GMT

Thanks Melita,

Do you happen to know why ArcGIS online basemap on a flexviewer application or javascript measure distance and area correctly? vs. when bringing the basemap into ArcMap. I'm still confuse as how that happens, it seems to measure correctly on a web browser vs. in ArcMap(unless you change the projection).

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

MelitaKennedy — Fri, 05 Sep 2014 15:50:28 GMT

They're returning the geodesic distance rather than the planar distance. The geodesic distance is the shortest distance between two points on the GCS ellipsoid surface. You can get the geodesic distance in ArcMap too. The Measure Tools supports it. I think it defaults to the planar distance if the data frame's coordinate system is a projected coordinate system. In the measure tool, it's an option on the far right of the tool.

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

TateM — Fri, 05 Sep 2014 21:49:53 GMT

I get the straight measurement distance, but what about area measurement (sq. km, acres)?

Take Lake Maurepas for example in South Louisiana, it is about 240 sq. km. But when bringing ArcGIS basemap into ArcMap with its native projection of WGS84 web Mercator auxillary sphere, the measurement come out over 310 sq. km.

But when changing the projection in the Data Frame into a UTM Zone 15N or a StatePlane, it come out to be about 240 sq. km. It also correctly show about 240 sq. km. for the lake on the Web browser when measured. I'm still a little confuse that in ArcMap it shows differently unless you change the projection. TIA

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

MelitaKennedy — Fri, 05 Sep 2014 22:33:26 GMT

ArcMap hasn't been updated to report geodesic-based areas yet, so to get an equivalent area calculation, you have to switch to a coordinate system that has less area distortion.

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

CharlyParra — Sun, 07 Sep 2014 18:16:38 GMT

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?

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

MelitaKennedy — Mon, 08 Sep 2014 16:03:48 GMT

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

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

CharlyParra — Mon, 08 Sep 2014 17:35:19 GMT

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.

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

TateM — Thu, 11 Sep 2014 01:07:24 GMT

Melita,

So somehow measuring area in a web browser on ArcGIS online basemap is correct. Do you know or can you elaborate on that a little more? What the web browser have that ArcMap doesn't have on the desktop? They are both WGS84 Web Mercator auxillary sphere. Does the web browser automatically transform into another projecton on the fly?

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

MelitaKennedy — Thu, 11 Sep 2014 17:40:49 GMT

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.

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

MelitaKennedy — Thu, 11 Sep 2014 17:43:29 GMT

Different teams manage the various APIs, Server, Desktop, etc. so each software can be at an different stage for implementing functionality. The Geometry Runtime library added support for calculating geodesic-based areas before the geometry library used in ArcGIS for Desktop did. The algorithm is internally converting to an equal area projection. We found that was more stable and flexible for different sized polygons than various algorithms that calculated geodesic-based areas directly.

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

Anonymous User — Fri, 06 Apr 2018 19:45:10 GMT

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!

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

MelitaKennedy — Fri, 06 Apr 2018 22:55:47 GMT

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.