Projection Workflow

[ChuckTurlington]

Hello,

I am using ArcMap 10.4.1.

After watching the ESRI video presentation titled "Introducing Coordinate Systems and Transformations", under the subtitle "What is Happening When We Project Data?", under Case 2: Each PCS contains a different GCS; describes the projection as Un-projecting PCS A1 to GCS A->Transforming Geographic Datum to GCS B->then Projecting to PCS B1 as shown graphically in the attached screen shot.   

After reviewing this information I am assuming that the ArcMap program is following this workflow, when the projection path is pre-defined.  If the path is not pre-defined, then manually following this method is a logical solution.  I have to ask if my understanding is correct; that the workflow presented may be a manual procedure if the projection path is not defined, following the above steps?

[DanPatterson_Retired]

Your screen shot is the case when the datum of the geographic coordinate systems.  ie. the case where you want to "project" from a utm nad 27 to a utm nad 83... the projection requires a datum transformation, which is the part at the bottom of the image, unproject from nad27 utm to a geographic coordinate system of a defined ellipsoid.  perform the transformation to a different geographic coordinate system with the new ellipsoid, and finally project it to a utm in the new datum.  If the ellipsoids are the same, then there is no datum transformation required

[ChuckTurlington]

Dan,

Are these steps performed automatically, or is the user expected to know what steps to perform in such a coordinate system change, as in the example screen shot ?

[DanPatterson_Retired]

it you go from utm nad27 to utm nad83, you should get offered a transformation option in the lower part of the dialog, if not, I would select one, being the sentient one .

I only work with projected data usually and I don't have anything in different datums to test..  you could also try a multistep utm27 to GCSNAD83 (or even GCSWGS84), and it should also want the datum change, then GCSNAD83 to UTM 83

[ChuckTurlington]

Dan,

Are there multi-steps outlined in a white paper for other coordinate reference changes such as WGS84 to NAD83, and for vertical transformations/re-projections such as MSL to NAVD88, or NVGD29 to MSL or NAVD88, etc.?  We get a variety of different coordinate references and datums, and often have difficulties getting a proper transformation/projection without a lot of time and headache.  It seems I may not be using the correct multi-steps to get the desired results.  Also, some other software like NGS tools, I believe do not perform these multi-steps automatically, so require some additional user knowledge of what the multi-steps are to get proper results.

[DanPatterson_Retired · Accepted solution]

transformations work in both directions ie nad83 <=> wgs84

As for vertical ones, I suspect the same, but Melita Kennedy‌ could probably confirm.

I prefer human-aware to automagic in any event

[MelitaKennedy]

So all transformations are bidirectional / reversible. They have to be to support raster reprojection/transformation (we project extent and some points to create the output raster, then fill it by projecting sections back to the original to get the cell values). 

I need to work on documents for the US and Canada for the horizontal side--NAD83 and its realizations/re-adjustments. 

Melita

[ChuckTurlington]

Melita,

Thank you for your support with all my questions. 

The subject of a multi-step procedure to transform and/or project a datum(s), is a very technical subject, with many different exceptions and rules I am learning.  I guess, if I do not get a transformation/projection path listed in the right hand panel in ArcMap, then I need to try a different "logical" step" until I get a path that can lead to a transformation/projection.   Or can I assume that I will always go from the projected datum to the geographic datum, transform to the new geographic datum, then project back to the new projected datum? 

[MelitaKennedy]

Yes, that's correct.