There’s a lot of geographic data out there and most of it was created using a spatial reference. However, people are still dealing with issues of not knowing the spatial reference for some of their data, which includes imagery.
I generally see these four problems:
- What is a spatial reference?
- I know the spatial reference information but how do I apply it to the data?
- The spatial reference exists but the data appears in the wrong location.
- I don’t know the spatial reference for my data.
Spatial reference, projection, coordinate system?
If you’re asking “What is a spatial reference?” then you should start with some of these other options:
- Training: Understanding Map Projections and Coordinate Systems
– Mapping Center: Map Projection Animations
– Help topic: Georeferencing and coordinate systems
-Help topic: What are map projections?
– 2010 Developer Summit: Understanding & Using Geometry, Projections, and Spatial Reference Systems
– Book: Lining Up Data in ArcGIS: A Guide to Map Projections
Applying the spatial reference
Defining a spatial reference (projection) for your data is not the same as projecting your data. When you define the projection you’re simply identifying the coordinate system that is used to place you data’s coordinates in the correct location. For example, there are a number of raster dataset file formats that use an associated world file to define coordinate and pixel dimensions for the upper left corner of the raster dataset. Most commonly you’ll find .tif and .tfw files, or .sid and .sdw, or .gif and .gfw files. You need to use the Define Projection tool, otherwise the coordinate defined in the world file may place the image in the incorrect location. This tool will create an .aux.xml file that gets stored with your raster dataset containing the spatial reference information.
When you’re projecting data, you are changing the coordinate values and sometimes the shape of your image (due to the warping of the pixels to another location). You can change the projection of your raster data using the Project Raster tool. If your imagery has no spatial reference you may have to georeference it using the Georeferencing toolbar.
Finding the spatial reference
If you’ve opened up the data’s properties in ArcGIS and you’ve examined the data’s storage location for any other associated files, such as a metadata file, and you still don’t know the spatial reference, where do you start? Or if your image is displaying in the wrong location, what do you do? These problems are the same for vector data as for raster data.
Hopefully, you know where in the world your raster dataset is supposed to be. This usually helps because there are common spatial reference systems for each country, state, or province.
Next, look at the coordinates. You can open the world file (it’s just a text file), or add the raster dataset into an empty data frame in ArcMap and examine the coordinates as you move your mouse around, or look at the coordinates in the Properties dialog box. If the y-dimension is between between -180 and +180 and x-dimension is between -90 and +90, these are longitude and latitude values so your data is using a geographic coordinate system. If it’s recent data you can probably use the WGS 1984 projection (or at least start there). If these values are larger numbers, then it’s probably a projected system and these units normally represent meters or feet. In the USA you’ll typically find your data in a State Plane coordinate system (often using feet) or in a UTM zone (using meters).
Unknown spatial references are a common problem, but using the steps described in the following should help:
- Help topic: Identifying an unknown coordinate system
– 2010 UC presentation: Using Map Projections and Transformations
The above topics recommend various ways to help determine the correct system. One way I would recommend is to add a basemap in ArcMap (which typically use WGS 1984 Web Mercator Auxiliary Sphere projection), then modify the projection of the data frame until your image appears in the correct location. For example, start by changing it to the UTM zone projection in your local area, then change it to the State Plane zone. You can also choose a State Plane in feet or meters. You may also need to modify the datum. For example a datum using NAD83 may be a few 100 meters from one using NAD27.
See the above links for more tips. But if you don’t know the spatial reference then you need to become a geospatial detective to identify what’s correct for your data. If you can’t find the answer you may have to use the Georeferencing toolbar and reproject the data.
Submitted by: Melanie Harlow