# About geographic transformations and how to choose the right one

By Aileen Buckley, Mapping Center Lead

You will often be prompted to select the geographic transformation when you are projecting data or setting the projection of a data frame in a map document. Here are some concepts that might help you understand what this is all about AND how to make the right selection.

First, “geographic coordinates” are expressed in terms of latitude and longitude. “Latitude” is the north-south angular measure from the equator to the point of interest. “Longitude” is the east-west angular measure along the equator from the prime meridian to the point of interest’s longitude. Assuming that the earth is a sphere, geographic coordinates are determined relative to the center of the sphere – these coordinates are called “geocentric latitude and longitude”. (See the figure at the right; all figures are from Map Use: Reading and Analysis, 6th edition, Esri Press.)

Second, we know the earth is not a perfect sphere – it is elongated along the equator due to centrifugal force – its “equatorial radius” is longer than its “polar radius”. (The figure below is drawn using the real difference in length between the two axes which is not noticable at this scale.)

Therefore, instead of assuming that the earth is a sphere, we can more accurately assume it is an “oblate ellipsoid of revolution” (a sphere that is slighted squashed in at the poles), which we usually just call an “ellipsoid”. Rather than use coordinates defined by the spheroidal approximation of the earth, we use geographic coordinates expressed as latitude and longitude that are referenced to a particular ellipsoid of a defined shape and size. These coordinates are called “geodetic latitude and longitude”, and they will be slightly different from the geocentric ones. (See the figures below; in the one at the right, the difference between the equatorial and polar axes is greatly exaggerated!)

Different ellipsoids are used to approximate the earth, based on slightly different definitions of polar and equatorial axes of the ellipsoid - that is, the size and shape of the ellipsoid.

Third, imagine that the ellipsoid that is used as the model of the earth is then “positioned” relative to the surface of the earth to fit a specific area or use. This “fitted ellipsoid” becomes the frame of reference for specifying point coordinate locations, and it is called a “datum”. You should now be able to see how coordinates are datum dependent!

We have said that the ellipsoid is positioned to fit a specific area or use.  If the ellipsoid is being used as the basis for a national or regional datum (i.e., frame of reference to define coordinates), it will be positioned to best fit the area of interest (like the North American Datum of 1927, or NAD27).  If it is used as the basis for a world datum, the ellipsoid will most likely be positioned relative to the center of the earth’s mass (like the World Geodetic System of 1984, or WGS84).

Fourth, the datum, along with the prime meridian and angular units comprise a “geographic coordinate system” (GCS) which is referenced to the 3D ellipsoidal approximation to the earth. style=”text-decoration: underline” This is used to specify locations defined by latitude and longitude and heights above or below the ellipsoid at any latitude, longitude location.

Fifth, a “projected coordinate system” (PCS) results when the GCS is flattened down onto a 2D surface (e.g., paper, computer screen, etc.)  A projected coordinate system is always defined relative to the GCS that it comes from, which in turn is based on a specified datum, which is defined in part by its ellipsoid.

SO – when you are asked to select the geographic transformation, you are being asked to choose which mathematical calculation will be used to convert coordinates referenced to one datum to coordinates referenced to another datum. This geographic or datum transformation is often embedded in the procedure to convert between coordinate systems, or in other words, the projection process. This process often involves more than one coordinate transformation. For example, let’s say you want to convert between two projected coordinate systems. This is what would happen in the projection process:

1. Define the PCS that your data are currently in. The PCS includes the GCS.
2. Unproject the data to geodetic latitude and longitude using the same GCS.
3. Transform the data to geodetic latitude and longitude using the new GCS.
4. Project the data to the new PCS using the new GCS.

You can see that when you select the PCS you want to use, you also need to select the geographic transformation (step 3 above) because there are multiple mathematical calculations that can be used to define how the coordinates will be converted to the new GCS. Note that even if you are converting from one GCS to another GCS, you still need to define the geographic transformation; however, the will be no need to convert from PCS to GCS and visa versa during this kind of projection process.

So how do you choose the geographic transformation that should be used? Here are two Esri Knowledge Base articles that can help you:

HowTo: Select the correct geographic (datum) transformation when projecting between datums. This article contains links to downloadable zip files (for different versions of the software) that contain a list of all available datum transformations and their appropriate geographic areas of use.

1. NAD_1983_To_WGS_1984_1 – for the entire North American continent.
2. NAD_1983_To_WGS_1984_2 – for the Aleutian islands.
4. NAD_1983_To_WGS_1984_4 – superseded by _5; this transformation method should no longer be used!
5. NAD_1983_To_WGS_1984_5 – for the 48 contiguous United States.

Note that geographic transformations work in either direction. For example, the transformation listed as NAD_1983_To_WGS_1984_5 transforms from NAD 1983 to WGS 1984, as well as from WGS 1984 to NAD 1983. When using the Project Tool, the geographic transformation is recorded in the metadata.

Thanks to Melita Kennedy, David Burrows, and Rob Juergens of the ArcGIS Projection Engine Team for helping me to make sure I said things correctly!

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1. mattfindley says:

Thanks. The graphics distinguishing geocentric and geodetic concepts was helpful. Any ideas on how to choose between lists of other transformations like NAD83 WGS84 that have several varieties? For example, the differences between the following transformation functions aren’t real clear (at least between 2 and 3):

Almost scared to ask, but what’s the significance of all the dx, dy, dz parameters that appear at the bottom of the dialog box when loading a transformation function?

• kreuzrsk says:

Like Mattfindley, I can’t figure out if I should be using 2 or 3 for Washington state.

2. maphew says:

this article needs just a few more pictures, these ones here for example: http://yukongis.ca/bin/view/Main/WebForum#nabble-p12172118 Since they came from ESRI literature to begin with, it is only natural they should be here too!

Since posting the above thread, I’ve also been asked several times for similar images for transformations for other parts of the globe.

• abuckley says:

Sorry, maphew — that link seems to be broken — maybe you can send the update?

3. Helenak says:

Dear Dr. Buckley,

Your explanation is much better than my GIS teacher.
Thank you very much.

Recently, I have a problem transferring GCS 1984 (unit: degree) to Behrmann (unit: km).

The projection was all right, but the unit was not.
Do you have any advice ?

Elaine

4. abuckley says:

Regarding the comment about the graphics that mattfindley wrote in about, you might be happy to find that the graphics for this article came from the Map Use book published last year, and that you can order the book with a fuller explanation and more graphics through ESRI Press. You might be even happier to find that you can download right off the web the set of PowerPoint presentations that contain EVERY graphic in the book on a separate slide! This is a fantastic resource for teachers, students and others — kudos to ESRI Press for sharing this fabulous resource with all of us!

5. abuckley says:

I’m copying this response from Margaret to Elaine’s question:

Projection files for projected coordinate systems that are installed with ArcGIS Desktop use units of foot_us, foot (the international foot), or meters. Using units of kilometers in a projection file is not advised, because the unit of measure is so large you can lose accuracy in your data. You can customize a projection file to use that unit, following the instructions in Knowlege Base articles 30583 or 32939, but we don’t recommend it. Instead, use the standard units of meters. You can then easily calculate length in the attribute table of the data with units of kilometers using the Calculate Geometry tool. You can also add a new field to the attribute table named LENGTH_KM for example, as a DOUBLE field, then divide the existing length in meters by 1000 to give lengths in kilometers.

6. xienchen says:

Regarding the question from mattfindley about differences between the transformation methods, the transformations with parameter values and areas of use are in the geographic_transformations.pdf file in ArcGIS installation > Documentation folder. You can also look in the “Geographic Transformations” section in the desktop help, online here: http://webhelp.esri.com/arcgisdesktop/9.3/index.cfm?TopicName=Geographic_transformation_methods

The section on Equation-based methods might also be useful.

7. abuckley says:

Regarding the question from mattfindley about differences between the transformation methods, the transformations with parameter values and areas of use are in the geographic_transformations.pdf file in ArcGIS installation > Documentation folder. You can also look in the “Geographic Transformations” section in the desktop help, online here: http://webhelp.esri.com/arcgisdesktop/9.3/index.cfm?TopicName=Geographic_transformation_methods

The section on Equation-based methods might also be useful.

8. heavymetal says:

Regarding the question from mattfidley about “what’s the significance of all the dx, dy, dz parameters”, the related concepts and illustrations are in the online help at this URL:

http://webhelp.esri.com/arcgisdesktop/9.3/index.cfm?TopicName=Equation-based_methods

9. abuckley says:

Regarding the question from mattfidley about “what’s the significance of all the dx, dy, dz parameters”, the related concepts and illustrations are in the online help at this URL:

http://webhelp.esri.com/arcgisdesktop/9.3/index.cfm?TopicName=Equation-based_methods

10. nisha says:

Hello, Dr. Buckley

I live and work in Minnesota.

I now know which method to use for NAD_1983_To_WGS_1984, but I am still unsure of which method to use for NAD_1983_HARN_To_WGS_1984. There are three options for my state.

I have a similar issue with NAD_1927_To_WGS_1984. For this method, I have two different options for my state.

Would you say, generally speaker, we should use the one with the biggest last digit number for the applicable area?

11. cmcglone75 says:

How can I determine which method of transformation and parameters are used in the geographic transformation files?
Where are they stored?
Wouldn’t it be a lot more useful to have this information available when actually doing the reprojection, ie, within the project dialog box?

12. abuckley says:

Nisha: Wondering if you can post your specific question on our Ask a Cartographer page — this is a great place to send us questions exactly like this. Here’s the link:

13. dbmcalpin says:

Some of the information in this blog is outdated. Be sure to check the hyperlinks text where available. The list of NAD83 to WGS84 conversions, for instance, is very different from a 1 June 2011 update:
http://support.esri.com/en/knowledgebase/techarticles/detail/24159.

14. yvonne1986 says:

• abuckley says:

There are a number of NAD27 to NAD83 transformations. You want to pick one that has been created specifically for the part of the world that your data are location in. On Mapping Center, click the ArcGIS Resources tab and on the More tab (http://mappingcenter.esri.com/index.cfm?fa=arcgisResources.more), download the Geographis Transformations zip file. Unzip it and you will find a Word document taht lists all the geographic transformation. Use the Find tool to search for NAD_1927. You will see all the NAD27 to NAD83 transformations and a short description of the area of the world that each was created for. From this list, you will know which geographic transformation to specify.

15. wabairo says:

I’d really appreciate if you can point me to someone at NGS that can help me with this:
I’m trying to figure out in ArcGIS 10.3:
Transform coordinates from:
The State Plane zone is PR_USVI

For example: if I have a dataset in which CRS is EPSG:32161 (NAD83(86)), meters
If the dataset is using meters: Which one is the correct transformation method?
If the dataset is using angles(lat-long): Which one is the correct transformation method?

What is the purpose of each of those in the list?
ArcGIS does not have NAD83(CORS96) for PR_VI. Why?
Why there is nad83 to wgs84_5? is this wgs84_5 only for conus? Why not wgs84_1?

16. kofilartey says:

i Finally found the correct coordinate system for some of my data, which i built using the custom option. However when i tried to project some shapefiles to this coordinate i had issues; Error invalid geographic transformation. When i go into the environment for the projection tool to set the parameters, my particular coordinates gives me NAD_1927_TO_NAD_1983_NADCON. When i tried to use this in the geographic transformation, i still got the error message. At the same time the geographic coordinates system i used was GCS_War_Office, and when i input that too i get the same error message. What should i do

17. oliver_burdgis says:

How do you choose when you are close to the borders of different transformations. For example, if transforming between Arc1950 and WGS84 there are many options (~10). The documentation in the ArcGIS install files (geographic_transformations.pdf) suggests using transformation 8 for Zambia. However, if transforming data in Northern Zambia close to the DRC border, ArcGIS automatically suggest transformation 7. In the past I’ve noticed Arc does not always suggest the most appropriate transformation (NAD27 to WGS84 it has suggested the wrong one before). So how should I decide? The difference is an East – West shift of circa 40m which is huge in terms of accuracy.

• mkennedy says:

The algorithm that calculates the sorted list of transformations is easy to fool. It’s using envelopes, rather than polygons, for one, and only knows the extent not that the data’s really in Zambia and not in DRC.

Melita
Sr. Product Engineer

18. wildnicole says:

Hi there,
I’m quite new to ArcGIS, and still learning about transformations. I am using ArcGIC10.2 and have a map with two coordinate systems: GCS GDA 94 and UTM GDA 94. I understand that ArcMap projects these on the fly, but I am wondering if I need to transform the GCS GDA 94 data to UTM GDA 94 to edit the data.
Thanks,
Nicole

• mkennedy says:

Hello Nicole,

The UTM projected coordinate system is also using GDA 94, so no transformation is needed. You can edit in either coordinate system. You may see a warning message when starting the editor–as long as you’re working within the UTM zone, you don’t need to worry.

Melita
Sr. Product Engineer

• wildnicole says:

Thank you very much Melita. That really helps me out
Cheers,
Nicole