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For the Data Frame Properties on a map in ArcMap, what is the difference between selecting a new coordinate system and using a Transformation?

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Sometimes this is referred to as a "datum transformation". In ArcGIS I guess the datum part of the phrase is implied. Here's the first thing google returns for me: earth-info.nga.mil/GandG/coordsys/datums/index.html I don't think Arcmap does any sort of vertical datum transformation (?) nauticalcharts.noaa.gov/csdl/learn_datum.html –  Kirk Kuykendall Jan 31 '11 at 15:49
    
its reaally very usefull for me i think its right we do transformation in Arc map if datums are different. –  user4152 Sep 7 '11 at 7:42
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3 Answers 3

up vote 9 down vote accepted

A transformation should be used to "transform" between two systems such as geographic coordinate system (GCS) and Projected Coordinate System (PCS). There are other instances for it's use also. Link #3 is the ESRI 9.3 help page that is pretty good about describing the difference.

Way too much detail for me

A little more Lay description

Just right
To choose the correct transformation see the esri help
and for the right application.

Transformation

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The transformations he's referring to, in the data frame properties, are geographic transformations, i.e. they transform from one GCS to another GCS, not from GCS to PCS. The latter are referred to as projections rather than transformations. You probably know this but I wanted to clarify for the sake of other readers. –  LarsH Jan 25 '13 at 15:20
    
@LarsH you would be incorrect. they do not only transform from GCS to GCS. PCS have datum also. Any reprojection done can use transfomation files if needed. –  Brad Nesom Jan 25 '13 at 17:07
    
Brad, if you could provide an authoritative reference, I would appreciate the correction. IIUC, PCS "have a datum" only in the sense that a PCS is based on a GCS, and the GCS has a datum. Geographic transformations can be involved in a reprojection if the reprojection includes unprojecting to one GCS, transforming to another GCS (that's the geographic transformation), and then projecting to another PCS. I'm looking at resources.arcgis.com/en/help/main/10.1/index.html#//… –  LarsH Jan 25 '13 at 17:24
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Currently, the projection engine in ArcGIS supports GCS-GCS transformations only. There are transformation methods that convert directly between two PCS or between PCS and GCS [that is not a projection algorithm] but we don't support them yet. I work on the projection engine. Does that make me an authoritative reference? b-> –  mkennedy Jan 25 '13 at 17:47
    
ok my bad. I had always wondered why arcmap puts in a default transformation when you select re-project from pcs to gcs (but the transformation reads backward.) as the image above shows. So I havene't used it in qute a while but I surely have data out there at old jobs that is re-projected wrong. –  Brad Nesom Jan 25 '13 at 18:29
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Any Projected Coordinate System has a Datum. If you are converting in between two different Coordinate systems, with different datums, you will need to use a transformation.

If the two coordinate systems are based on the same Datum, then a datum transformation is not needed.

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In the ArcGIS world, a 'projection' converts between a geographic coordinate system (GCS) and a projected coordinate system (PCS). A geographic transformation, aka datum transformation, aka transformation, converts between two geographic coordinate systems.

When you set the data frame's coordinate system, you are defining the coordinate system (including the unit of measure) that you'll be working in. Any layers that are in a different coordinate system will be automatically projected (or 'unprojected') to this coordinate system. If a layer's coordinate system uses a different GCS, then you may see a warning that they're different. Whether you see the warning or not, you should decide whether you need to set a geographic transformation. Neglecting to do so when it's necessary can lead to data being up to a few hundred meters offset.

ArcMap only sets one automatically: NAD_1927_To_NAD_1983_NADCON, which transforms between NAD27 and NAD83 in the lower 48 states.

Std Disclaimer: I work for Esri.

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This is the best answer. –  LarsH Jan 25 '13 at 15:21
    
Note: Esri dropped the NADCON transformation as a default in ArcGIS 10.1 SP1. –  mkennedy Jan 25 '13 at 20:08
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