Geographic Coordinates

[ChuckTurlington]

Hello,

I am using Arc Map 10.4.1.

I am at odds with the terminology of "Geographic" coordinate and inquire for a better understanding.  I have heard many interpretations of this term.  Such as: a datum is geographic if one is using lat-long coordinates.  Which I have always assumed belong to a "round" earth.  However, there are graticules associated with a flat map, that I believe, sometimes have lat-long, such as along the edges of a quad map.  When viewing graphics set to a coordinate system like Arc Map, I understand that the geographic coordinate is actually flat, but allows 3d data to be added to the map.  But with a cad application like AutoCAD, the opposite is true, one cannot add data to a geographic coordinate, but rather, must be in a projected one such as a state plane, to add 3d data.  I find the subject somewhat confusing and look to Esri for some explanation of this "Geographic" concept.  So I have ot ask, is it possible to have geographic coordinates for a flat map, as well as a 3d datum, and is the term really just another way to refer to a lat-long coordinate format?

[RobertBorchert]

A geographic coordinate system is one designed to allow users to better map the entirety of the Earth or very large geographic areas.  

Generally speaking your projected coordinate systems are a better for for smaller defined areas. 

Our G&T spans 3 different UTM zones. We used to use the one in the middle but it dramatically skews data in the other zones.  We work with a number of small distribution systems and I would build their systems in the appropriate projected system.  Depending on the shape and size of the system.  

One system built their own GIS and SHOULD have use UTM.  But they insisted on using State Plane because a 3rd party solution was not compatible with UTM.  However they spanned 2 different State Plane systems.

We finally settled on WGS_1984_Web_Mercator_Auxiliary_Sphere for everything because it keeps data more uniform across large areas and is very good for web gis.

[DanPatterson_Retired]

Chuck

Spherical coordinates in math and other disciplines

Geographic in Geography and now GIS.

Some history with reference to earth based and celestial as well

Spherical coordinate system - Wikipedia 

I never used Geographic Coordinate System in the early days of GIS (70's).  Spherical was used to differentiate it from planar

[MelitaKennedy · Accepted solution]

Hi Chuck,

I personally use "geographic coordinates" to mean latitude-longitude. Sometimes that could include ellipsoidal heights (heights/depths relative to the spheroid/ellipsoid surface). 

I have seen others, including in Esri documentation, use "geographic coordinates" to mean the positions of a thing so the coordinates could be in a "geographic" or a "projected" coordinate system. 

When Esri (and others) display data that's stored in latitude-longitude as a 2D map like in ArcMap, it usually using Plate Carree or pseudo-Plate Carree where the degrees of the latitude-longitude values are treated as if they're linear values. The regular Plate Carree projection just scales the degrees into meters--that is, the math of the projection is not complicated.

A graticule is a set of points or lines that show where the lines of latitude and longitude are in that map's projected coordinate system. So you actually have to convert the latitude-longitude into the map's coordinate system in order to place them properly.

If the display is truly 2D, then you can add 3D data to it, but usually not see the 3rd dimensional data except by querying a feature's properties. If you instead have a 2.5D or 3D display, then you can see the 3D properly placed and usually pan and tilt around it. It can get strange because much 3D GIS data is not using the same reference surface for the horizontal and vertical coordinate systems. That is, if the vertical coordinate system (VCS) is for orthometric/normal/geoidal/gravity-related/MSL/LLWS/etc., the "zero" surface for those is not the same as the projection surface of a PCS, nor a GCS's ellipsoidal surface. so that can cause some confusion. 

Maybe that's why AutoCAD doesn't want to display latitude-longitude plus gravity-related heights/depths, but this is pure speculation on my part as I haven't used AutoCAD. 

Melita

[ChuckTurlington]

Thanks Melita and everyone for your thoughtful responses.  It appears that the NGS tools will convert from northing-easting (projected grid) and provide lat-long in a true geographic format (non-projected location).  I am surprised to find engineers requesting lat-long in conjunction with northing-easting values.  This makes me think of mixing coordinate formats, not sure it makes any sense to do that.  I would assume that most engineers probably do not know what they are asking for when requesting coordinates in northing-easting, as well as, lat-long format.  Is it a common practice to provide both formats for a coordinate location, one projected and one non-projected as geographic?

[DanPatterson_Retired]

At least where I am from, the legal description of property provides the corners of lots in degrees min sec, probably less likely to undergo datum redefinitions

[MelitaKennedy]

I don't know but I know US surveyors seem to divide between lat-lon, a local system, or state plane. Control networks are likely to report latitude-longitude plus ellipsoidal or geoidal/gravity-related heights and/or 3D Cartesian (XYZ or ECEF - earth-centered-earth-fixed). GNSS natively reports lat-lon-h (or maybe XYZ), not projected coordinates.

Melita