Your Guide to Canadian Datum Transformations and ArcGIS Online (Part 3)

Welcome to the final post in our Canadian Datum Transformation series! So far, we’ve been learning why it’s important to apply the correct datum transformation method to your map prior to publishing to ArcGIS Online and what transformation method to use depending on the projection of your data. Today, our focus is going to be on data that is defined with a datum of NAD 83 Original, and we’ll cover not only what transformation method to use, but where to access the various grid shift files for Canada and just as importantly, where to store them and how to use them. Read on for all the details!

You guys. We did it! We made it to the final post in our datum transformation series! The emotions are real, but before we celebrate, we should probably, you know, actually make it through the most complicated part Part 3. You can read Part 1 here and Part 2 here, but the Cole’s notes version is this: Canadian data is usually in a coordinate system of either NAD 83 Original or NAD 83 CSRS. When you are preparing to publish your data for use in ArcGIS Online, you need to apply a datum transformation so the software knows how to correctly project your data into Web Mercator Auxiliary Sphere, which is the coordinate system used by the default basemaps in ArcGIS Online. Without applying any transformation (or applying an incorrect transformation), you could see a misalignment of up to 2 meters between your data and the basemaps available in ArcGIS Online.

In the previous posts in this series, we learned that if your data has a projection of NAD 83 CSRS, you should be using the NAD_1983_CSRS_To_WGS_1984_2 transformation method (read Part 1 for all the juicy details). In the case where your metadata indicates the data is in NAD 83 Original but the data is actually in NAD 83 CSRS, you should use either the ‘NAD_1983_To_WGS_1984_5’ or the ‘WGS_1984_(ITRF00)_To_NAD_1983’ transformation methods to transform the data appropriately (read Part 2 for all the juicy details).

Today, we’re going to talk about what to do if your data is truly in NAD 83 Original. First, let’s look at what transformation methods are possible to select from the drop-down list:

There are a few different options to choose from when reprojecting from NAD 83 Original to WGS 84.

Going into detail for each of these options:

• NAD_1983_To_WGS_1984_5 – We already learned in of this series that this method is actually a transformation from NAD 83 CSRS that just happens to be called generic NAD 83 Original. So, it is not suitable for data truly in NAD 83 Original

• NAD_1983_To_WGS_1984_1 – This is a 0 transformation. You get the impression you are doing something to your data, but there is no shift applied at all. So, this is also not a suitable choice.

• NAD_1983_To_WGS_1984_4 – this transformation method does not extend to Canada’s geographic area, so it’s also not a suitable choice.

Right, so we’ve got three different options and none of them are suitable choices for data truly in NAD 83 Original. Where does that leave us? Have no fear, I have a plan! What we need to do is apply a two-step transformation that first projects our data (or data frame) into NAD 83 CSRS and from there we can transform it to WGS 84 (NAD_1983_CSRS_To_WGS_1984_2 transformation method).

Transforming between NAD 83 Original and NAD 83 CSRS is an NTv2 (National Transformation version 2) grid based method. At this point, I will note that depending on your geographic area, the shift between NAD 83 Original and NAD 83 CSRS could be negligible (a few cm). Depending on where you are working, it may not be worthwhile to go through the trouble of shifting your data. If you are interested in understanding how great the shift is between the two different datums in your area, please refer to the NTv2 tool that NRCan provides. With this tool, you select the grid you are interested in, type in some sample coordinates from your data (in either geographic or UTM) and press ‘calculate’ to the shift. If the difference is not significant for you, simply move forward with the NAD_1983_To_WGS_1984_5 transformation method to transform your data (or dataframe) into WGS 84.  

Assuming you do need to proceed with the transformation to NAD 83 CSRS, grid shift files (.GSB) are used to shift the data. For us here in Canada, the grid shift files do not come with the software, so it’s up to us to a) obtain the necessary grid shift file and b) place the grid shift file in the correct location so the software knows to access it. Unfortunately, there is no NAD 83 Original to NAD 83 CSRS national grid shift file that works for all of Canada. Currently, grid shift files are maintained by regional authorities. In the table below, for each province/territory, I’ve listed out the name of the transformation method to use, the name of the grid shift file and where to acquire the grid shift file (note: this table only includes grid shift files to transform between NAD 83 Original and NAD 83 CSRS):

* In some cases, there is no out-of-the-box transformation method in the software that is associated with the corresponding GSB file. If that is the case for the region you are working in, what you need to do is create your own custom transformation method. This sounds complicated, but there’s a nifty geoprocessing tool that basically does all the work for you. Side bar: back in the olden days (read: pre 9.2), this tool didn’t exist! If you wanted to use a grid shift file for a transformation method that didn’t exist in the software, you had to go in the back end and edit some crazy GEOGTRAN file….it was a whole thing (I feel like I’m telling you I used to walk to school uphill both ways, but there you have it). If you need help using this tool, please reach out to our incredibly intelligent folks in Technical Support.

** Some regions within Canada have not yet developed a grid shift file to transform between NAD 83 Original and NAD 83 CSRS. If you find yourself in this position, I’d recommend applying the NAD_1983_To_WGS_1984_5 transformation, even though that method is meant for data actually in NAD 83 CSRS. It’s not ideal, but with no grid shift file for your area, the choices are limited.

One additional note about the table above. Some regions may not have a grid shift file to transform between NAD 83 Original and NAD 83 CSRS, but they do have other grid shift files that may apply to your data. For example, in New Brunswick, it is common to have data defined in the ATS 1977 datum, and the province provides a grid shift file to transform between ATS 1977 and NAD 83 CSRS. So, while this post focuses specifically on NAD 83 Original and NAD 83 CSRS, bear in mind grid shift files to transform between other datums in Canada do exist. A full list of all grid shift files for Canada can found via NRCan’s NTv2 calculator by clicking the ‘Select Grid’ link:

Click on ‘select grid’ to see a list of all available grid shift files in Canada.

Once you have obtained the appropriate grid shift file for your area, you need to place it in a very specific location, so the software knows where to find it. For ArcMap users, that location is here:

• C:\Program Files (x86)\ArcGIS\Desktop10.6\pedata\ntv2\canada
  (note: you will likely need to create the ‘canada’ folder yourself)

For ArcGIS Pro users, the location you are after is here:

• C:\Program Files\ArcGIS\Pro\Resources\pedata\ntv2\canada
  (note: you will likely need to create the ‘canada’ folder yourself)

At this point, I’ve thrown a lot of information at you, so it might be helpful to walk through an example together. Let’s pretend we’re working with data in Alberta and using ArcGIS Pro – here’s what our workflow is going to look like:

1. Using the table above as a reference, I will contact Geoff Banham (Geoff.Banham@gov.ab.ca)

2. Once I have a copy of the file, I will rename it to AB_CSRS.gsb and store it here:

   • C:\Program Files\ArcGIS\Pro\Resources\pedata\ntv2\canada

3. In Pro, I will set the coordinate system of my map to WGS 1984 Web Mercator Auxiliary Sphere

4. In Pro, I will navigate to Map Properties> Transformation and select the following transformation method:

   • NAD_1983_To_NAD_1983_CSRS_3 + NAD_1983_CSRS_To_WGS_1984_2
     (note: the NAD_1983_To_NAD_1983_CSRS_3 method will only be visible once you have the appropriate grid shift file stored in the correct directory)

5. Publish the service

Select a two-step transformation to go from NAD 83 Original --> NAD 83 CSRS --> WGS 84

How about another example? This time let’s pretend we’re working with data in Ontario and using ArcMap – here’s what our workflow is going to look like:

1. Using the table above as a reference, I will contact either Morgan Goadsby (morgan.goadsby@ontario.ca) or Geoff Clark (Geoff.Clark@ontario.ca) and request the ‘ON83CSv1.gsb’ file

2. Once I have a copy of the file, I will store it here:

   • C:\Program Files (x86)\ArcGIS\Desktop10.6\pedata\ntv2\canada

3. I will use the Create Custom Geographic Transformation tool to create a custom transformation method (you get to choose the name of the transformation, but I generally like to go with something straight forward like: NAD_1983_To_NAD_1983_CSRS_ON)

4. In ArcMap, I will set the coordinate system of my map to WGS 1984 Web Mercator Auxiliary Sphere

5. In ArcMap, I will navigate to Data Frame Properties> Coordinate Systems tab> Transformations and select the following transformation method:

   • NAD_1983_To_NAD_1983_CSRS_ON + NAD_1983_CSRS_To_WGS_1984_2
     (note: the NAD_1983_To_NAD_1983_CSRS_ON method will only be visible once you have the appropriate grid shift file stored in the correct directory and create the custom transformation method)

6. Publish the service

Select a two-step transformation to go from NAD 83 Original --> NAD 83 CSRS -->WGS 84

And there you have it! Soooooo simple…am I right? No, I recognize this is a complex topic (hello - this is already the third post in this series!), but I hope all this information helps lay a clear path forward as people are increasingly turning to ArcGIS Online to showcase their data. As always, if you have any questions or feedback, get in touch by posting in the comments section below!

This is the last article of a three part series. The other posts can be found here:

• Part 1
• Part 2