Does the Coordinate system of a data frame in ArcMap have to match the coordinate system of at least one of the layers in the table of contents?

I have found that if defined it will take on the coordinates of the previous layer. If both undefined it will stay undefined? I am not sure if the answer is yes or no. I believe it is yes because I have found this excerpt.

By default a data frame (the map display area) takes on the same coordinate system as that of the first spatial data layer added to it. It is important to realize this. But,assuming that all of the layers in your data frame have their coordinate systems defined properly, you can set the data frame to display your data in any desired coordinate system, regardless of the coordinate system of the layer you bring in first.

To me it is a bit confusing. So if the coordinate systems are all defined then the answer would be no, I assume. Can anyone clarify this for me?


The default action for ArcMap is to set the Coordinate System of the Data Frame to be that of the First layer that you load in. Any layers loaded after that will be projected on the fly to match this coordinate system.

If the first layer loaded in has an undefined coordinate system, then the Data Frame coordinate system will be undefined as well. This means that there will be no on-the-fly reprojection of layers. They will simply be placed at the coordinate positions of their features.

It is Very Important to choose a coordinate system for your Data Frame. You also need to make sure that the coordinate system for each of the layers is defined, as this will enable On-The-Fly reprojection of all of your layers, and ensure that they overlay correctly. If you do not know the coordinate system, here are some tips for Identifying an unknown coordinate system.

Regardless of the default action, you can set the Coordinate System of the Data Frame, to be anything you want. It DOES NOT have to match the coordinate systems of any of the layers that you have loaded in.

You could, for example, have your Data Frame set to : GCS_WGS_1984 - Datum: D_WGS_1984

All of the layers that are loaded in, could be using projected coordinate systems like State Plane or UTM, with differing units. They would all be reprojected on the fly to overlay in the proper latitude/longitude location.

Having said all of this, I think there is a generally accepted practice to set the Coordinate System of your Data Frame to match that of the majority of the layers you are using, as this will reduce the amount of processing that has to happen, and also decrease the amount of distortion in either shape or area due to reprojecting.

I encourage you to read the ESRI Help files about "What are Map Projections". They have a lot of really useful reference information.

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  • It's a good thing the data frame's coordinate system doesn't have to match that of one of the layers: none of my data is stored in the Dymaxion projection ;) – nmpeterson May 15 '12 at 20:51
  • @nmpeterson this is true, though I fear that projection will guarantee distortion in both shape and area, and most likely, perception as well! – Get Spatial May 15 '12 at 20:55

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