Geographic Maps

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09-09-2018 11:08 AM
ChuckTurlington
Occasional Contributor II

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. 

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DanPatterson_Retired
MVP Esteemed Contributor

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.  

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DanPatterson_Retired
MVP Esteemed Contributor

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.  

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ChuckTurlington
Occasional Contributor II

Dan,

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

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DanPatterson_Retired
MVP Esteemed Contributor

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 

ChuckTurlington
Occasional Contributor II

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?

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DanPatterson_Retired
MVP Esteemed Contributor

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)

MelitaKennedy
Esri Notable Contributor

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

ChuckTurlington
Occasional Contributor II

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?

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MelitaKennedy
Esri Notable Contributor

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

ChuckTurlington
Occasional Contributor II

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?

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