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Aren't all coincident points created equally?

September 13 2007 | 0 comments
Categories: Map Data

This question was posed to me. If I input into ArcMap a point of latitude and a point of longitude in a layer and put the identical two points into a second layer, shouldn't they lay directly on top of each other? If they don't would you be able to tell me why? I don't have any other information. Is it that the layers are not in the same projection or state plane area?

Mapping Center Answer:

The reason they wouldn’t align is because they are not using the same coordinate system (projection), particularly that they are not using the same datum. To fix that, you need to set a datum transformation for your data frame. To do that, open the data frame properties window and click on the Coordinate System tab, then click the Transformations button. Choose the transformation that makes sense given the coordinate systems for your data.

The most common instance of this is somebody who creates a dataset that uses NAD83 for its datum and then imports GPS data that are stored in WGS84.

If your datasets both used the same datum, the on-the-fly-projection feature of ArcMap would make sure the points align properly; only when the datasets’ datums are different and no transformation has been specified will the data not align.

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