Esri Mapping Center - Ask a Cartographer

Geographic Transformation

July 21 2011 | 2 comments
Categories: Map Data

Hello, I am having trouble with locating the .gtf that I created using Create Custom Geographic Transformation tool. I am converting a data set from an unknown projection to NAD 83 16N. I already have the formulas for my coordinate transformation and did twelve points by hand, but I would like to create a .gtf I can use to transform all of my data to UTM. Even though I can find the .gtf I have created when I go to computer\Documents and Settings\\Application Data\ESRI\ArcToolbox\CustomTransformations, it doesn't show up when I use tools like "define projection" and then go to the custom list. Is this happening because I don't already have an existing projection?

Mapping Center Answer:

There could be two things happening. Because the in data doesn't have a coordinate system, the tool can't match it against the transformation. What did you use for the input coordinate system for the custom transformation? If the tool allowed nothing, that could be a bug!

The other possible issue is that we block geographic transformations from being used for non-GCS data. So if you create a custom GT and use a PCS for the input or output coordinate system, it'll really be the underlying GCS that is used.

You might want to look at the spatial adjustment or georeferencing documentation to see if that will help.

that's right posted by Jill Kleiner on Jul 25 2011 11:26AM

Yes I was setting my coordinate system as Unknown in the input box of GT. What should I set my coordinate system as if I would still like to create a custom GT for my data? It would take a really long time if I tried to rectify it all by hand. If I already know the XY axis translation and rotation would I be able to set a temporary coordinate system just for the GT?

Try Spatial Adjustment toolbar instead posted by Aileen Buckley on Jul 28 2011 12:53PM

A geographic (datum) transformation only converts between two geographic coordinate systems—in latitude-longitude. It sounds like you've calculated the parameters for a 2D (rather than 3D) conformal or affine transformation. A 2D conformal/affine transformation will convert between two projected coordinate systems like a "local" or unknown grid and a UTM zone. A geographic transformation is also only used when actively, converting data between the two coordinate systems like in the Project Tool or in ArcMap.

You might instead want to look at the Spatial Adjustment Toolbar, where you can quickly enter some links, and then edit them (via the link table) to match your control points.

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