ArcMap is doing it's best to display all three data sets by "projecting on the fly" to put them all into the same spatial reference. There is a tiny Humboldt County at the top and there is actually one that is so small it is invisible at the bottom. You may have already seen a window like this and you probably will in the near future. Then, click on the little earth button to zoom to see all the data sets. Now, let's load all the data sets into ArcMap together at the same time. This shows that it is very easy to have a missing or incorrect spatial reference and ArcMap will never recognize it - you have to! Also note that I did not get an error loading this data. Note that the coordinate value is huge (because it is actually a UTM coordinate) and the units indicate "Decimal Degrees". Humboldt County with an incorrect spatial reference. In this case, ArcMap will have popped up a dialog letting you know the data set had an "Unknown Spatial Reference" and down at the bottom you may notice that the units are "Unknown Units".įigure 3. Humboldt County in UTM Zone 10 North, WGS 84 but without a spatial reference. Humboldt County properly referenced as UTMįigure 2. Especially note the coordinate value and units in the lower right.įigure 1. select "File -> New" before loading each one) in ArcMap. Watch what happens when we load each of these into an empty data frame (i.e. For one file, I removed the spatial reference (just delete the PRJ file) and for another I've changed the spatial reference to Geographic WGS 84. Let's take the following examples, I've taken a boundary file for Humboldt County that was in UTM Zone 10 North WGS 84. However, if one has a missing or incorrect spatial reference, ArcMap will still display the data, just in the wrong place. If all the data sets loaded into ArcMap have the correct spatial reference, everything is fine. This sounds good but creates a whole series of other issues. ArcMap includes a feature called "Project on the fly" where it will try to project data sets to the same spatial reference. Having missing or incorrect spatial references can cause major problems for analysis and cartography. Spatial References and ArcGIS Incorrect and Undefined Spatial References in ArcGIS
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