Continuing the topic started in the November Geospatial Frequently Asked Question (G-FAQ), in this month’s edition I conclude the conversation on geographic transformations, projections and more in ArcGIS. In this G-FAQ, I focus on the operational side of geographic transformations in ArcGIS versus the ‘theory’ we covered last month. Specifically, we will look at the three PDFs Esri delivers with ArcGIS about this topic, and offer advice on completing projections, re-projections and geographic transformations.
As a quick reminder, the November and December G-FAQ’s address this core set of questions:
What exactly is a geographic transformation? How do the various Projections and Transformations toolbox functions differ? What advice can you offer when working with shapefiles, rasters and geographic transformations?
ArcGIS Documents on Projections and Geographic Transformations
To start off the discussion this month, let’s have a look at three reference documents that are included with ArcGIS (at least with ArcGIS 10.x, and perhaps with ArcGIS 9.x as well). You can find them in your program files on the hard drive with your ArcGIS install. On my 64-bit laptop, they are located here:
C:Program Files (x86)ArcGISDesktop10.1Documentation
Depending on how you installed ArcGIS, they might be stored in a slightly different location. If you cannot find them, fear not as you can download the PDF’s below:
- Geographic Coordinate Systems– this document provides a wealth of information on the geographic coordinate systems available in ArcGIS as well as the locations each can be used. It also contains information on the horizontal and vertical datums that are available.
- Projected Coordinate Systems– this document contains information on the projected coordinate systems you can use in ArcGIS as well as the locations where they apply.
- Geographic Transformations– this is an important document which shows you the various geographic transformations offered in ArcGIS. The most important item to take note of is the Accuracy column. This shows how accurate the geographic transformation is in millimeters – a value of 999 represents an unknown accuracy value. The other important columns are the geographic location that each transformation is valid over. I will refer to this document in more detail later in this G-FAQ.
Projections and Re-Projections in ArcGIS
Now let’s delve into ArcMap and look at the three projection and re-projection Toolbox functions. These three functions are located in the Toolbox under Data Management -> Projections and Transformations. The Define Projection function is used to set the projection of your data if, and only if, it is completely un-projected. Refer to last month’s G-FAQ for tips on determining if your data is projected or not before you use this tool. If your data is not projected, the functionality of this toolbox is straight forward as you will need to select the correct projected or geographic coordinate system and then the correct datum. In order to determine the correct projection, you will need to look at the metadata files that (hopefully) accompany your geospatial data.
If your data is already projected and you need to change this from one system to another, then you will need to use the Toolbox functions under sub-menu Feature -> Project for shapefiles and Raster -> Project Raster for rasters. There are few differences in the steps you take to re-projecting shapefiles and rasters. First, you will need to pick the coordinate system and datum you wish to re-project your geospatial data into. If you convert from a projected to geographic system (or vise versa) with the same datum, a geographic transformation is not required. In all other cases, you will need to apply a transformation – I will discuss this step in some detail in the following section. Also keep in mind that you will need to know the correct zone number if you plan to convert to a projected coordinate system.
As a quick side note, when re-projecting rasters, it is a common practice to include the latitude and longitude value of known locations, called ground control. This will improve the horizontal accuracy of the re-projected rasters. Unfortunately, ArcGIS does not allow you to apply photo-identified ground control in the Project Raster toolbox function.
Advice on Geographic Transformations in ArcGIS
Now comes the part of the lesson that all of you have so patiently waited for, choosing the correct geographic transformation! For those ArcGIS 10.x users, you will notice that the geographic transformation is more often than not auto-populated with a value. Arc picks this value based on the Accuracy column I mentioned above. It does this by cross-referencing the source and output datums with the data’s latitude-longitude ranges to pick the geographic transformation with the lowest Accuracy value (i.e. the most accurate transformation).
For those users of ArcGIS 9.x, you should refer to the PDF included with Arc 10.x as in past versions, this accuracy column was not automatically assessed. You will want to select the transformation that is valid in your specific location and converts between the correct source and output datums. This is a large list of transformations so be sure to look through the entire list before making your selection.
Here are the tips I can offer to help you avoid the pitfalls I have encountered with geographic transformations.
- While Arc 10.1 has almost always chosen a geographic transformation for me, there have been instances when this has not happened. As of yet, I cannot determine what causes this. Either way, the solution is simple: grab the third PDF above and find the correct transformation on your own.
- If you are re-projecting multiple geospatial datasets over the same area, be sure to use the same geographic transformation on all of the datasets to avoid any possible issues with the inherent inaccuracies of each.
- Perhaps the most important tip I can offer is the possibility that you are not using the correct transformation as you are converting between two older revisions of a common datum. See the November G-FAQ for more details on datum revisions. Unless you have detailed metadata about your geospatial datasets, there is a very good chance that you would never know this is occurring. One way you might notice an incorrect transformation is regular (and likely very minor) horizontal displacement across an entire dataset. So for instance, you notice a 3-meter shift to the north when you compare all ten known latitude and longitudes locations (sometimes called ground control points) to your data. If you see this, I suggest you experiment with several alternative geographic transformations to minimize the regular shift. In some cases, this will solve your issue, and in other cases it will not.
Until our next edition of G-FAQ, happy GIS-ing!
Do you have an idea for a future G-FAQ? If so, let me know by email at firstname.lastname@example.org.
Find Out More About This Topic Here:
- Esri ArcGIS Resources – About Geographic Transformations
- Esri ArcGIS Resources – Map Projection Animations
- Esri Knowledge Database – How To Article 24159
- National Park Service – Datums & Coordinate Systems
- University of Washington Course Notes – ESRM 250
Brock Adam McCarty