ARCMAP-TRANSFORMATION AND PROJECTION

[ChuckTurlington]

Hello,

I am using Arc Map 10.3.  When I change coordinate systems by selecting Data Management Tools->Projections and Transformations->Project-> fill-out dialogs for XY and Z coordinate to change from State Plane Nad83 Us Feet, NAVD88 to a Geographic WGS84, WGS Ellipsoid; I believe ArcMap is performing both a transformation to a different horizontal datum and a projection from a flat grid based state plane system to an elliptical geographic system.  Is my understanding correct or am I missing something.

[curtvprice]

You only get a geotransformation between datums if you specify it. Fortunately, the most appropriate one is provided in the pick list for the parameter (if one exists).

Until recently, Z transformations were not included, just X and Y.  To get Z transformations, in 10.5 and later, you still need to do a special install of a separately downloaded coordinate geotransformations installer for ArcGIS Desktop, see: Information on the Coordinate Systems desktop install?

[curtvprice]

By the way, Chuck, queries to GeoNet often get better answers (and have more usefulness in general) when posted to an appropriate forum. I shared this to the Coordinate Reference Systems‌ forum.

[ChuckTurlington]

Curtis,

Thanks for forwarding to the proper forum, I was unfamiliar.  Will look forward to their responses.

[DanPatterson_Retired]

see you other post as well Chuck

[curtvprice]

Here's how to move things when you find you posted to the wrong space:

https://community.esri.com/docs/DOC-2258 

[MelitaKennedy · Accepted solution]

At 10.3, we aren't doing any vertical transformations. Some support was added to 10.4.1 and Pro 1.4, with more coming in Pro 2.0. 

For your question, "I believe ArcMap is performing both a transformation to a different horizontal datum and a projection from a flat grid based state plane system to an elliptical geographic system", this isn't quite true. 

The data is being converted from a projected coordinate system, plus a gravity-related vertical coordinate system. The target is a geographic coordinate system and ellipsoidal heights based on the geographic coordinate system. At 10.3, what you would actually get is WGS 1984 for the geographic coordinate system, and the vertical/z values are still referenced to NAVD88. Vertical coordinate systems are...complicated. Most are only loosely tied to a horizontal (geographic or projected coordinate system) and even though usually because there's a transformation method that uses a grid/file of offsets and you have to have gcs connected to it to interpolate the offsets.

In this case, at 10.5, we would convert the NAVD88 values to nad83 ellipsoid heights using the Geoid12b geoid model, then convert from NAD83 + NAD83 ellipsoidal heights to WGS84 + WGS84 ellipsoidal heights. If the target VCS was WGS84 geoidal heights AKA EGM2008 heights, the third step would be to convert the WGS84 ellipsoidal heights to geoidal using the EGM2008 geoid model.

Melita