topic Re: Geographic Maps in Coordinate Reference Systems Questions

Geographic Maps

ChuckTurlington — Sun, 09 Sep 2018 18:08:29 GMT

Hello,

I am using ArcMap 10.4.1.

I have a base map that has the "GCS_WGS_1984" geographic coordinate reference system, it is my understanding that this reference system is referring to a 3d coordinate system. However, if I export the map, I assume the map is actually projected to a flat image, especially since it appears on a flat computer screen? If it is projected, what projection is used because it is not referred to as a projected coordinate but as a geographic type, which is somewhat confusing how the exported map is meant to be interpreted.

Re: Geographic Maps

DanPatterson_Retired — Sun, 09 Sep 2018 18:17:24 GMT

It is 3d in the sense that there can be height values assigned. WGS84 is datum used by a geographic coordinate system ( longitude and latitude ). There will be no 3D component unless elevations are assigned to the geometry as well as the coordinates.

Re: Geographic Maps

ChuckTurlington — Mon, 10 Sep 2018 09:19:00 GMT

Dan,

Does that mean that I cannot add elevation to anything in a projected state plane coordinate?

Re: Geographic Maps

DanPatterson_Retired — Mon, 10 Sep 2018 09:24:07 GMT

Chuck your can create PointZ, PolylineZ and Polyline Z geometry

for instance

Create point and multipoint features—ArcGIS Pro | ArcGIS Desktop

or you can add elevations to the features table and use those values to 'extrude' the features for work in 3D

A reference from pro... but there are equivalent functions in arcmap

Convert a map to a scene—ArcGIS Pro | ArcGIS Desktop

Re: Geographic Maps

ChuckTurlington — Thu, 13 Sep 2018 18:33:20 GMT

Dan,

In reference to your above discussion "It is 3d in the sense that there can be height values assigned. WGS84 is datum used by a geographic coordinate system ( longitude and latitude ). There will be no 3D component unless elevations are assigned to the geometry as well as the coordinates." When does the concept of Projection come into play, if datums are actually flat projected type, when one is performing a "Transformation / Projection?

Re: Geographic Maps

DanPatterson_Retired — Thu, 13 Sep 2018 18:53:25 GMT

datums have a 3d component if you consider that they define an earth 'radius', it just doesn't go beyond establishing elevations above that elliptical surface. Transformations are needed when the datum differs. The difference could be due to a difference in the description of Earth over time (ie NAD27 vs NAD83) or a datum difference associated with a locational application (WGS84 to NAD83... the former is applicable on a global scale, the latter on a continental scale)

Re: Geographic Maps

MelitaKennedy — Fri, 14 Sep 2018 00:17:20 GMT

Chuck,

There are two types of vertical data. The first is ellipsoidal heights (or depths) which are relative to the geographic coordinate system / datum's surface. So, latitude-longitude-h. If you project the data you now have easting-northing-h where the h (z) values are still relative to the ellipsoid surface. They're not relative to the projection planar surface.

The second type are heights/depths relative to a gravity-related surface. US examples are NGVD29 and NAVD88. These are only loosely tied to the horizontal datum (NAD27/NAD83/etc.). For instance, z values relative to NAVD88 could use NAD27 or NAD83 as the geodetic datum/coordinate system.

NGVD29 is a leveling datum. Someone set a zero location on a coast somewhere based on local mean sea level and then calculated the relative heights everywhere else in the country. Newer vertical datums use gravity measurements, GPS, other surveying techniques.

Melita

Re: Geographic Maps

ChuckTurlington — Fri, 14 Sep 2018 12:14:37 GMT

Melita,

Thanks again for your support, but I have to ask: the basemap imagery associated with the GCS_WGS_1984 geographic datum, inside ESRI ArcMap, is actually flat or do I assume the basemap has earth curvature in the imagery?

Re: Geographic Maps

MelitaKennedy — Fri, 14 Sep 2018 19:47:05 GMT

Chuck,

If it's in WGS84, and not Web Mercator (EPSG:3857), it's being treated as if it's flat. Depending on the application you're using to interact with it, you will usually get distances and areas calculated based on the curved surface, not the flat image. Is that what you were asking?

Melita

Re: Geographic Maps

ChuckTurlington — Fri, 14 Sep 2018 19:52:39 GMT

Yes, thank you Melita. Is the flat imagery, treated with any particular projection? The Web Mercator (EPSG 3857) is not projected, but retains the datums elliptical shape?

Re: Geographic Maps

SimonKettle — Mon, 17 Sep 2018 10:37:05 GMT

A bit late to the game but I'm not sure if this blog might of help to you: https://community.esri.com/groups/coordinate-reference-systems/blog/2014/08/14/mean-sealevel

Re: Geographic Maps

MelitaKennedy — Mon, 17 Sep 2018 18:53:06 GMT

Chuck wrote: Is the flat imagery, treated with any particular projection?

No. Some tools/functions now know to handle distances and areas differently if the coordinate system is a geographic one. If the data's using a projected coordinate system, the tools/functions normally do 2D/planar calculations. Web Mercator is projected, but online apps / web maps will usually not do planar distance calculations because there's so much distortion in a Mercator map projection.