Thursday, January 13, 2011

ArcGIS: Define Projection vs Project

In ArcCatalog, there are three projection related tools in ArcToolbox. These tools caused me confusion in the beginning, and they seem to cause others confusion as well.

Define Projection - This tool does not change a projection. It changes the metadata describing the current projection of the dataset. You should only use this tool if the projection is currently described as unknown or is known to be incorrect. With this tool you are defining or describing the dataset by saying "Hey, those points are in this projection."

Project - This tool operates on features to change the current projection from one defined projection to another. For example, if you have a point dataset in a UTM projection, each point has a pair of numbers describing that point. When you use this tool to change its projection, each number is recalculated to reflect its equivalent value in the new projection.

Project Raster - This tool operates on rasters to change the current projection from one defined projection to another.


Also, if you have a dataset and the projection is currently described as unknown, and despite all your Google searches, emails and phone calls, you can't determine what it is supposed to be, and you can't find an equivalent dataset with the projection properly defined, you can use the following method as an act of desperation to define it. Please, make sure you record in the metadata that the projection was "guessed at" so future users will know that there is uncertainty about whether the defined projection matches the projection that the coordinates belong to.

1. Create a new document in ArcMap.

2. Add a reference dataset with a defined projection (it cannot be defined as unknown, or be potentially incorrect)

3. Add your dataset with the unknown projection (make sure it is defined as unknown)

4. Right click on the map and select data frame properties

5. In the data frame properties dialog, select the Coordinate System tab

6. Remember, this is detective work. Go back and look at the point values themselves if needed. Do the coordinate values look like they are in meters or degrees? What is the state plane projection for this location?

7. Select a coordinate system that is your best guess for what the projection might be, click ok to exit the dialog

8. As you change the projection for the data frame, the reference dataset will continue to be in the correct location on the map by reprojecting on the fly (coordinates are recalculated in memory for the map, but the dataset on disk remains unchanged).

9. The dataset with the unknown projection will start hopping all over the place. No on the fly projection is happening for this dataset. It is placed on the map using its unaltered coordinate values.

10. After repeating this process, and your unknown dataset aligns perfectly with your reference dataset, you have LIKELY found the correct projection. I emphasize likely, because there is still uncertainty, and you should note this uncertainty in the metadata.


5 comments:

  1. Thanks for posting! I finally understand defining a projection vs projecting.

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  2. Thank you for this!

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  3. Hi! Your definitions seem very clear. I'm a beginner with GIS and it really helps!

    I was trying to use the near tool with my data and realized that it was giving me the distance in decimal degrees. After doing some searching online and on the Esri resource center I found that I would need to change my coordinate system to a Projected Coordinate System and after some more searching I needed to change my coordinate system to State Plane (US Feet) California zone 6 for Orange County, California. I used arc catalog to change the coordinate system using properties (I also tried defining the coordinate system through the toolbox and and using the project tool) of all of my shapefiles and though ok this should work. Unfortunately when I used the measure tool to test out my new file, it said the county was less than a mile across. This person seems to be having a similar problem: http://forums.esri.com/Thread.asp?c=93&f=1149&t=91668
    I'm really confused and not sure what I'm doing wrong. :( Any help would be greatly appreciated.

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  4. Changing the projection in properties is the same as running the Define Projection tool. In both cases, they should only be used if the coordinate system is undefined or wrongly defined. To project geographic coordinates, use the Project tool

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  5. hey, my lec gives this kind of question.
    what is meant by a GIS project?. is this the correct answer?

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