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Of all the GIS questions I field, coordinate system related questions are the most frequent. The following is typical:
I have a shape file of land parcels from Bonneville Co. and am trying to overlay that onto the County NAIP imagery. For some reason, when I have the same coordinate system and datum assigned to all layers, the NAIP and shape files are projecting at different scales and not even coming close to lining up. I’ve tried everything in my limited arsenal. If you have any idea of what’s going on, please let me know.
The problem here is not a lack of understanding of the software, but misunderstanding concepts behind assigning and translating a coordinate systems.
I've found language translation to be the most accurate analogy for making sense of coordinate systems in ArcGIS.
Suppose you're traveling on a ferry in the Baltic Sea (why not?). A frazzled tourist is running from person to person animatedly asking a question. Fortunately, you have a web browser on your phone and you call up Google Translate to assist. You are two dropdown listboxes away from understanding:
- First, you must correctly identify what language they are speaking.
- Second, you must indicate what language to which you want the information translated (most likely English)
You hand your phone over and they pound quickly into the keys:
missa ovat wc
You've selected English as the output for your benefit, but the original language requires a guess, so you randomly start trying local tongues.
Swedish suggests Ovat has bad aim...
Ms. Latvia haz Water Closet...
Ah, the Finn has to pee!
Ah, but how do we ask the Dutch crew?
Projections in ArcGIS present exactly the same problem. ArcGIS needs to “know” what coordinate system you want everything to be in, and it needs to know what coordinate system the different layers are in to begin with, so it can successfully translate. In some situations (generally where the datum differs) it is a best practice to permanently re-project the data to match the rest of your data. The most insidious errors occur when you incorrectly guess one or more coordinate systems and receive gibberish in response.
Idahos to the Rescue
So what to do when layers don't play well with others? Out here we bring on the Idahos. Or "Projection Finder" as I call it professionally. This is a layer I made that is projected in IDTM83 and displays where fifteen common projections fall to use in identifying projections.
How to use this layer
- Open a new map and add the Project Finder layer first. This is important as starting a new map and adding a layer sets the default projection of the data frame to match the first layer added.
- For the layer you want to determine a projection for, first make sure it has no projection defined. There are two ways you can do this: in Windows Explorer delete the corresponding .prj file or in ArcCatalog right-click on the layer, select Properties, select the XY Coordinate System Tab and spank the Clear button.
- Once the coordinate system is nuked, add the layer to the map.
- Zoom to the layer to to see in which projected Idaho it falls.
- Now you can go to Toolbox > Data Management Tools > Projections and Transformations > Define Projection and do just that.
- Once it is defined, remove it and re-add it to your map and it should move to fall inside IDTM83 since it now has a projection defined it is reprojecting on the fly to match the data frame. This is a good thing.
Oliko se hyvä sinulle? Osta minulle olutta.
- Projection_Finder.zip - Idahos Shapefile... if you are on the IDFG network you'll find it already in the SDE_Layers folder or in the Add Data dialog of IDFG GeoTools
- NAD 27 Idaho TM (Meters).prj - Projection File for ITM27
- NAD 83 Idaho TM (Meters).prj - Projection File for ITM83
Copy the projection files to the State Coordinate Systems folder for your ArcGIS installation if they are not already present. This location is typically (where DesktopX.X is your ArcGIS version number):
C:\Program Files\ArcGIS\DesktopX.X\Coordinate Systems\Projected Coordinate Systems\State Systems
Posted by idfg-bthomas at 12/03/2008 in
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