Welcome to part 2 of your guide to Canadian Datum Transformations! In Part 1 of this series, we learned why it’s important to apply the correct datum transformation method to your data prior to publishing to ArcGIS Online, and also what transformation method to use if the data has a projection of NAD 83 CSRS. In Part 2, we kick off a discussion about what to do if the data has a projection of NAD 83 Original. Read on for tips on how to tell if your data is truly in NAD 83 Original, and what transformation methods you should be working with.
Welcome to part 2 of your guide to Canadian Datum Transformations! In Part 1 of this series, we reviewed the current datum for Canada, and also learned what datum transformation to use if your Canadian data has a projection of NAD 83 CSRS (spoiler alert: it’s NAD_1983_CSRS_To_WGS_1984_2). In Part 2, we’re going to kick off a discussion about what you should be doing if your data has a projection of NAD 83 Original. I wish this topic was straight forward and as simple as choosing the appropriate datum transformation method, but sadly, it’s not (in my defence – I did provide a warning in part 1 that things were going to get more complex). So, maybe grab a fresh cup of coffee before diving into the details below!
To start, let’s review why it’s important to apply a datum transformation method prior to publishing and using your data in ArcGIS Online. If you don’t apply any datum transformation method (or, you apply the wrong transformation for the area you are working in), there will be a misalignment of 1-2 metres between your data and the ArcGIS Online default basemaps. This is because the default basemaps have a projection of WGS 84 Web Mercator Auxiliary Sphere, which is a different datum than most Canadian data uses. A misalignment of this size will be visible at a scale of 1:2000 and better. If, however, you apply the correct datum transformation method to your map prior to publishing, the data will line up properly, and you will not see any misalignment.
The first thing you will want to do is to verify the current projection of your data. You can do this by checking the layer properties of your data in either ArcMap or ArcGIS Pro:
The layer properties of this data indicate the datum is NAD 83 Original.
Here’s where things get tricky: sometimes, the data is defined as NAD 83 Original, but it’s actually in NAD 83 CSRS. Yep, you read that correctly. The metadata can be incorrect and gives people the wrong information about their data. This discrepancy leads to scenarios where people are “double transforming” their data – i.e., data that is already in NAD 83 CSRS (but defined as NAD 83 Original) is being transformed to NAD 83 CSRS. The unnecessary transformation can put the data up to 1 metre off from where it should be sitting, depending on where you are in Canada (in Thunder Bay, the discrepancy between NAD 83 Original and NAD 83 CSRS is about 70cm, while in Vancouver, the difference is about 10cm).
We’ve seen situations where data is incorrectly defined as NAD 83 Original all across the country. The remainder of this post will cover how to confirm if your data is truly in NAD 83 Original, as well as how to move forward if your data is not defined correctly and should actually be defined as NAD 83 CSRS. The third post in this series available next month will inform you on how to work with data truly in a coordinate system of NAD 83 Original (this will include information about how to transform from NAD 83 Original to NAD 83 CSRS).
In order to confirm whether your data is actually in NAD 83 Original, your best option is to load a default basemap into your map and take a look at how things line up without any transformation applied at all.
Test to see how your data lines up against a default basemap with no datum transformation method applied.
If you notice a misalignment between your data and the basemap, similar to what is displayed in the image above, select the ‘NAD_1983_To_WGS_1984_5’ transformation method from the dropdown list and apply it to your map. Did your misalignment disappear?
This transformation method is similar (same method and parametres) to the ‘NAD_1983_CSRS_To_WGS_1984_2’ transformation method you should use when projecting data from NAD 83 CSRS to WGS 84. See, the ‘NAD_1983_To_WGS_1984_5’ method is actually a transformation from NAD 83 CSRS that just happens to be called generic NAD 83 Original. So, if you apply the ‘NAD_1983_To_WGS_1984_5’ transformation method to your map and your misalignment disappears, we can safely assume that the metadata is misleading, and that your data should actually be defined as NAD 83 CSRS.
After applying the ‘NAD_1983_To_WGS_1984_5’ transformation method, the misalignment with the basemap is resolved.
Full disclosure: if you look up information about ‘NAD_1983_To_WGS_1984_5’ transformation method, you’ll notice that most of the Esri documentation states the area of use for this method as the United States. This could be misleading because the transformation does work for Canada. Furthermore, documentation from EPSG states that this transformation method is deprecated, which could imply that this method will be removed in future releases of ArcGIS. As an alternative, you could run the same test by applying the ‘WGS_1984_(ITRF00)_To_NAD_1983’ transformation method to your map. This transformation will give you almost identical results as the NAD_1983_CSRS_To_WGS_1984_5 method (within 1cm):
If you’ve confirmed that the ‘NAD_1983_To_WGS_1984_5’ transformation method resolves the misalignment, simply use the process outlined in Part 1 of this series to set the appropriate transformation method in your map prior to publishing to ArcGIS Online. A quick note: if you see any strange behaviour when testing the transformation method (for example, applying the ‘NAD_1983_To_WGS_1984_5’ transformation method resolves the misalignment in some areas of your dataset, but not in others), make sure to notify us of such instances on the Community Map of Canada feedback tool.
If applying the ‘NAD_1983_To_WGS_1984_5’ transformation method does not resolve the misalignment you notice with the basemap, we can assume your data is in NAD 83 Original. You’ll need to stay tuned for the next instalment in this series coming in May. Until then, if you have any questions, please get in touch by posting in the comments section below!