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I have two shapefiles with different projections. When I load both into ArcMap I get the 'Geographic Coordinate Systems Warning' when the second is loaded into table of contents but they both align properly with one another. If I export the second one using 'export data' than choose to export it using the coordinate system of the data frame, this exported feature has the same projection as the first shapefile and aligns with it properly. But if I project the second shapefile using the toolbox I have to choose a 'Geographic Transformation' and this output doesn't properly align with the first shapefile.

Can anyone explain the difference between these to methods to change the coordinate system of a shapefile?

Why would I used the projection toolbox if it has to preform a Geographic Transformation and shift it retaliative to other features as opposed to changing the data frame projection and exporting it?

nmtoken
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Ian Schelly
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  • i may be wrong but from my understanding when you use the project tool you are projecting/transforming the the way the coordinates are calculated based on the datum. a geographic transformation is changing the underlining datum(mathematical approximation of the earths shape) – ziggy Sep 29 '16 at 17:26
  • check out this link it explains it way better than I can http://gis.stackexchange.com/questions/664/whats-the-difference-between-a-projection-and-a-datum – ziggy Sep 29 '16 at 17:27
  • and watch this video! best explanation I have seen https://www.youtube.com/watch?v=Z41Dt7_R180 – ziggy Sep 29 '16 at 17:27
  • When you get the Geographic Coordinate Systems Warning it may be default applying a geographic transformation. Check your transformations dialog when you next load this shapefile, and use that transformation in the Project tool – Midavalo Sep 29 '16 at 18:07

2 Answers2

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By default, ArcMap does not apply any geographic/datum transformations. It will reproject between two projected coordinate reference systems, but ignores any differences in the underlying geographic coordinate reference systems.

If two layers line up, without you setting a transformation, they are already at least using the same geographic coordinate reference system, or two that are so close that it doesn't matter.

The Project Tool enforces a geographic/datum transformation. When you set a transformation, it was applied and moved the data's coordinates.

If you set the same transformation in ArcMap, you would see the same offset.

So, for this data, use the version exported from ArcMap.

Note: if a transformation is set in ArcMap, it will be used when exporting the data as well.

mkennedy
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The simple and concise answer is that 'Reproject' actually transforms or re-maths the locations of features. Exporting in data frame's projection merely adjusts source metadata for the output .shp so that projection=data frame. Credit goes to u/sudocookie on Reddit.

Ian Schelly
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