A common problem in using GIS data arises when different data sets are in different coordinate systems. If you add two or more data layers that should overlap to your GIS session, and they show up in two different places (or your screen appears to show a blank map), this is most likely the reason. ArcGIS can handle data in different coordinate systems as long as the projection/coordinate system of each data set is defined in a way that the software can understand. Typically this means the data set has a .prj file in addition to all the other files that make up the data set (e.g., parks.prj). When you pull up data in different coordinate systems, ArcGIS tries to use the information in the various .prj files to project all the layers into one projection so that they all overlay each other.
Unfortunately, most data sets do not have their projection/coordinate already defined, even though they are in fact projected. That means that the software can't figure it out, and data layers that should overlay one another end up in two different places. This is a confusing concept for students new to GIS, so read carefully:
Every data set is in some coordinate system, either a geographic coordinate system (using latitude and longitude expressed in decimal degrees - e.g., 30.52 degrees north and -99.25 degrees west) or a projected coordinate system (e.g., State Plane).
But until recently, some widely used GIS software like ArcView did not have the ability to explicitly define the coordinate system of each layer - it was implied during the creation process but no information travels with the file to tell us or the computer about the coordinate system. So there was and is lots of GIS data out there with no defined coordinate system. The City of Austin is an example. All of the city's data layers are in the State Plane, NAD 83 (feet), Texas Central coordinate system. When you use this data, all the layers show up in the same place and overlay each other correctly, regardless of whether they each have a .prj file defining the coordinate system explicitly.
However, when you bring up data from other agencies in other coordinate systems you will begin to have problems, including the classic "Hey, where did the data go? Why isn't it overlaying right?"
For example, TNRIS has a hillshade image available. The image file is in the geographic coordinate system (latitude and longitude, decimal degrees). In ArcGIS, this is referred to as the GCS coordinate system. But so far there is no function to explicitly define the projection of an image with a .prj file. The software does recognize the GCS (the only coordinate system it recognizes immediately). But if you bring up a data layer in a different coordinate system and that lacks an explicitly defined projection (i.e., no .prj file included), the software will be unable to overlay them properly.
A similar thing happens with the digital orthophoto quarter quads from TNRIS (DOQQ). In this case the orthophotos are in the UTM NAD83, zone 14, coordinate system. But again, there is no explicitly defined file accompanying them. So frequently the software puts them in the wrong place.
For the tutorial data set, I have explicitly defined almost all of the City of Austin data sets (I missed a few, but better to test you that way). This means that in addition to the other files making up each shape file, you will see a .prj file.
When you first add data to an ArcMap data frame, the data frame sets itself to the coordinate system of the first data set to load. If the first data set has a .prj file, the data frame takes on the coordinate system specified in that .prj file. If the first data set has no defined coordinate system, the data frame coordinate system remains unknown.
You can always tell what coordinate system your data frame is in by right-clicking on the data frame name in the ArcMap table of contents and choosing Properties. Then go to the Coordinate System tab. You will either see coordinate system parameters or you will see that it says unknown:
Now listen up!
If the first data set to load has no .prj file, you will get a message that "One or more layers is missing spatial reference information. Data from these layers cannot be projected."
That is not a problem as long as all your data layers are in the same coordinate system. But once you start using data layers in two or more coordinate systems you will have problems because the software does not have the information it needs to project them into one system.
What to do?
The first thing to do is figure out why you are having problems. Typically there is one of several scenarios occurring (see below), but it may not be obvious which one. In any case, at least one of your data sets is probably undefined in terms of its coordinate system. To know which one (or more) it is, you can right-click on each layer to get its properties, and click on the Source tab. In the Data Source portion of the box you will see information about the coordinate system. It will say "undefined" or "unknown" if there no coordinate system has been defined. This layer is probably at least one of your culprits - there may be several of them. Next, look at the following scenarios and figure out which one applies to you.
|What to do||
Data sets are in different coordinate systems, and they all have defined projections (.prj files or other coordinate system information present).
ArcMap should be able to project all the data into one view.
|You shouldn't have to do anything - it works!||
Not all data layers may be projected accurately if they are based on different datums. Data layers may appear offset by a significant distance. This is a bigger problem than we are covering in this tip sheet.
|B||All your data sets are in the same coordinate system, but are not explicitly defined.||The data will overlay each other accurately because they are all in the same coordinate system, even though ArcGIS doesn't know what that is. However, you won't be able to measure or set up a good map scale on a layout because ArcMap doesn't know what units the coordinates are using.||If you know what the coordinate system is, you should use the Data Frame Properties-Coordinate System tab to define the coordinate system for ArcMap (typically that means choosing one of the Pre-defined, Projected coordinate systems, in our case, that would usually be State Plane or UTM). This will allow you to measure and set up a map scale on your layouts.||To establish units for measuring and for the scale, you can also simply set the map and display units in the Data Frame Properties- General tab. The map units must be the units of the coordinate system the data is actually in. The distance units can be the units you want to use for measuring or scales.|
You have data sets in two different coordinate systems. All data sets in one coordinate system have .prj files, but the data in the other coordinate system does not. However, you know the coordinate system for the undefined data sets.
Data that should overlay each other may show up in two different places. If you go to a full map extent, the screen may appear blank (because ArcMap is trying to interpret the coordinate system as one large x-y axis).
|Use the Data Frame Properties-Coordinate System tab to define the coordinate system for the data frame. Choose the coordinate system of the data set that is undefined. Typically this means choosing one of the Pre-defined, Projected coordinate systems, in our case, that would usually be State Plane or UTM).||
This is a very common case when using the DOQQ orthophotos from TNRIS together with City of Austin data. Define the coordinate system of the data frame to be Pre-defined - Projected-UTM-NAD83-Zone 14N.
|D||You have data sets in two or more coordinate systems. There are data sets in at least two coordinate systems without defined coordinate systems.(lacking .prj files). You know the coordinate systems for all your data sets.||Data that should overlay each other will show up in two different places.||You have to define the coordinate systems for each data set. To do this, you can either use ArcToolbox.or ArcCatalog.
In ArcToolbox, go to Data Management Tools-Projections-Define Projection Wizard (choose the right tool depending on whether the data set is a shape file, coverage or grid).
In ArcCatalog, follow the directions in the ArcGIS Desktop Help under ArcCatalog-Working with Shape Files - Defining a Shape File's Coordinate System (or the equivalent instructions for coverages or grids)
|There is not as yet any standard function in ArcGIS to define a projection for an image file.|
(up a creek without a paddle)
You have data sets in more than one coordinate system, there are no .prj files, and you have no idea about what the coordinate systems are.
Data that should overlay each other will show up in two different places.
|There is nothing you can do in ArcGIS until you find out what the coordinate system for each data set actually is. Make phone calls, send e-mails, pull your hair out, cry, find a new career...||
With experience, when you pull up a data layer without a defined coordinate system and no information about what the coordinate system is, you will start to recognize the coordinates (look in the lower right corner of the ArcMap screen to see the coordinates of the cursor as you move it around). For example, if you are looking at Austin area data and you see coordinates like: 3,156,142 and 10,083,099, these are Texas State Plane coordinates in feet.
Warning: if you start to recognize coordinate systems like this, it is a sure sign that you have been spending way too much time in the lab...
Basic advice on coordinate systems
The easiest way to get everything to work properly in ArcMap is to make sure that every layer you will be using has a defined coordinate system. The easiest way to do this is using ArcToolbox. But prior to doing this you must know the coordinate system of the data set, either through its metadata or from information given to you by some other source. If you do not know the coordinate system, you cannot define it.
Remember, your data set is always in some coordinate system! It is up to you to figure out what coordinate system it is, and if it is undefined, define it accordingly. You cannot define a coordinate system simply by what you would like it to be - you need to define the coordinate system it is actually in!
Two more warnings:
When using ArcToolbox to define projections, you cannot have the data up in ArcMap. Close out of ArcMap before going through the following steps.
This process creates a .prj file for your data set(s). Because it is writing a file, you must have write access to the folder where the data set is located. E.g., you cannot write to a CD unless you have made it writable, and you cannot write to a read-only folder. Make sure you are writing to a folder with write access.
Start ArcToolbox and go to Data Management Tools-Projections-Define Projection Wizard (choose the right tool depending on whether the data set is a shape file, coverage or grid). Note there is no tool for image files like .tif or .jpg
Warning - do not use the Project Wizard - this one is for when you want to project a layer with a defined projection into another projection
Double-click on the tool to start it.
Select the data set or sets for which you want to assign one coordinate system and press Add. You can select more than one data set, and select from different folders, but they all need to be in the same coordinate system (e.g., you have six shape files in Texas State Plane Central and two in UTM Zone 14 - only select the six or the two, not all of them).
In the Select Coordinate System dialog box, press Select Coordinate System
In the Spatial Reference Properties dialog box, you can press either Select to choose from a list of coordinate systems, or you can press Import to import coordinate system information from another data set already defined with the coordinate system. In this example, we will use the Select method, so press Select.
You now have to select the coordinate system. Assuming your data is projected (i.e., not in latitude/longitude coordinates), double-click on Projected Coordinate Systems.
Choose the next appropriate folder - typically for us in the Austin area, this is either State Plane or UTM. This example will assume State Plane, so double-click on State Plane.
Choose the appropriate datum (NAD) and units. For City of Austin data, this is typically NAD 1983 (feet). For data from other areas, you will need to know what is the correct choice. In this example, we use NAD 1983 (feet) so double-click on that.
Now you have to choose the state and zone within the state. For Austin, this is NAD 1983 StatePlane Texas Central FIPS 4203 (Feet).prj (the central zone of the Texas State Plane coordinate system based on the 1983 North American Datum, with coordinates expressed in feet).
The information is added to the Spatial Reference Properties dialog box. Check that the name is correct, and press OK.
The information is loaded to the Select Coordinate System dialog box. Press Next.
The information is summarized. Press Finish.
If you get error messages, it is typically because the layer is open in ArcMap (in use by another application) or you are trying to write to a read-only folder.
Bring up your data in ArcMap or ArcCatalog to see whether you can find the coordinate system information (under the Metadata tab in ArcCatalog - Spatial section)
If your data still does not overlay properly, you may have defined the wrong coordinate system. Check your information and try again - you can overwrite existing definitions using these same steps.
Piece of cake, eh?