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What is NAD83 (CSRS)?

NAD83 (CSRS) stands for North American Datum 1983 (Canadian Spatial Reference System). It is an updated version of NAD83 that incorporates improved accuracy using GPS measurements.

IMPORTANT: Most do not need to be concerned with the level of precision discussed here. But if data aren’t lining up and CSRS is being used, this might help.

Why Was CSRS Introduced?

The original NAD83 (1983) was based on ground survey data and early satellite measurements. Over time, more accurate GPS technology revealed small errors in this datum. While the GPS datum is updated regularly, there are still regional differences and when using high-precision GNSS receivers this comes into play. Mostly, the CSRS deals with local events moving the earth like melting of the permafrost, earthquakes and plate tetonics.

To improve positional accuracy across Canada, Natural Resources Canada (NRCan) introduced NAD83 (CSRS), refining coordinate positions with GPS data.

Key Improvement:

CSRS corrects positional errors in NAD83 using precise geodetic measurements.
Positions can shift by tens of centimeters to over a meter compared to the original NAD83.

Differences Between NAD83 and NAD83 (CSRS)

Feature	NAD83 (1983)	NAD83 (CSRS)
Based on	Ground surveys & early satellite data	GPS data with higher accuracy
Accuracy	Less precise due to older methods	Highly precise, better for modern applications
Shifts	May be off by up to 1m	Improved positioning (better within ~10 cm)
Usage	Older GIS datasets may still use it	Required for modern Canadian mapping & GPS

How Does CSRS Affect GIS Work?

If you mix NAD83 (1983) and NAD83 (CSRS) data without a proper transformation, your layers may not align perfectly.
CSRS is used in surveying, GPS applications, and modern mapping in Canada because of its improved accuracy.
GPS data uses NAD83 (1983) as its base, so data will differ significantly compared to NAD83 (CSRS).

Understanding NTv2 `.gsb` Grid Shift Files in GIS

When working with maps in GIS, we often reproject data between coordinate systems. A grid shift file (.gsb) is a special dataset used to improve the accuracy of this transformation—especially when moving between different versions of a datum.

Why Do We Need Grid Shift Files?

Most GIS data is tied to a datum, which defines how we model the Earth’s shape. Sometimes, a country or region updates its datum (e.g., from NAD27 to NAD83) to improve accuracy. However:

These datums are not perfectly aligned; shifting between them requires adjustments.
Simple mathematical transformations (like Helmert transformations) introduce errors in some areas.
Grid-based transformations (using .gsb files) account for local distortions, improving accuracy.

NTv2 grid shift files provide a higher-accuracy transformation by considering local variations in the Earth’s shape. They are essential for precise GIS work—especially in surveying, land management, and engineering.

How Does a `.gsb` File Work?

It contains a grid of known coordinate shifts between two datums.
Instead of applying a single mathematical formula, ArcGIS looks up the correct shift for each location.
This provides more precise transformations, often improving accuracy from meters to centimeters.

Analogy

Imagine you have an old and new road map:

A simple transformation assumes all roads shifted by the same amount.
A grid shift file recognizes that some roads moved more than others and corrects for that locally.

Example: NAD27 to NAD83 (CSRS) in Ontario

If you transform data from NAD27 to NAD83 (CSRS) in ArcGIS, you need the Ontario NTv2 grid shift file (.gsb).
Without it, ArcGIS may use a less accurate transformation, introducing errors of up to several meters.
By adding the .gsb file, ArcGIS uses local grid-based adjustments instead of a one-size-fits-all formula.

How to Use `.gsb` Files in ArcGIS Pro

Download the .gsb file from a trusted source (e.g., Ontario GeoHub, direct link below). Each province updates their own files so you must grab it directly from the province covering the area working data in. There are some national files for NAD27 to NAD83 CSRS transformation. The most common needed datum transformation is for GPS to CSRS, which is NAD83 to NAD83 CSRS.
With ArcGIS Pro not running, Place the file(s) in the correct folder:
- C:\Program Files\ArcGIS\Pro\Resources\pedata\ntv2\canada (for all users). If the canada directory does not exist, create it.
Choose the correct transformation in ArcGIS Pro (e.g., ON83CSv1.gsb). There is no harm in adding multiple, but to keep it simple ideally only add what you actually need.

Installing NTv2 files for CSRS datum transformations

The NTv2 Grid Shift files must be installed configured in ArcGIS Pro for CSRS datum transformations.

1. Download and Install the NTv2 Grid Shift Files

ArcGIS Pro requires NTv2 files (.gsb) to be placed in a specific directory. First download them from (Ontario) at https://geohub.lio.gov.on.ca/documents/7d26e16ba42040d7a4ae66a4e02d16a0/about. Note each province seems to update these themselves.

Three provincial files that are included cover all of Ontario. They are the ON27CSv1.gsb (Ontario NAD27(74) to NAD83-CSRS(1997)) file, the ON76CSv1.gsb (Ontario NAD27(MAY76) to NAD83-CSRS(1997)) file, and the ON83CSv1.gsb (Ontario NAD83 (Original) to NAD83-CSRS(1997)) file.

Option 1: Install for All Users

Copy the downloaded .gsb files to:
```
C:\Program Files\ArcGIS\Pro\Resources\pedata\ntv2\canada
```
(You may need administrator rights to create the folder and paste files here.)

Option 2: Install for a Single User

If you don’t have admin rights, copy the files to:

   C:\Users\YourUsername\AppData\Roaming\Esri\ArcGISPro\pedata\ntv2

(If the ntv2 folder doesn’t exist, create it.)

2. Use the NTv2 Grid in ArcGIS Pro

After installing the .gsb files, ArcGIS Pro can use them in coordinate transformations.

Method 1: Set Transformation in a Map

Open ArcGIS Pro.
Click on the Map tab and go to Properties.
Under Coordinate Systems, find Transformation settings.
Choose a transformation that references the NTv2 grid shift (e.g., NAD83(CSRS) to NAD27 using an NTv2 file).
If the NTv2 grid is properly installed, it will be listed as an available option.

Method 2: Set Transformation in a Geoprocessing Tool

For geoprocessing workflows (e.g., Project Tool):

Open the Project tool (Data Management Tools > Projections and Transformations).
Set the Input and Output Coordinate Systems.
Click Environments (at the bottom of the tool).
Scroll down to Geographic Transformations and select an NTv2-based transformation.

3. Verify NTv2 Grid Usage

To check if ArcGIS Pro is using the NTv2 grid:

Open Geoprocessing > Project tool and select your dataset.
Under Geographic Transformations, look for a transformation with an NTv2 reference.
If you don’t see it, ensure the .gsb file is in the correct location.

Important Notes:

Exporting a layer with an NTv2 grid-defined PRJ file will not automatically apply the datum transformation (NTv2 grid shift) when assigning the projection to other datasets.

PRJ Files Store Only the Coordinate System

A .prj file contains the coordinate system definition (horizontal & vertical reference), but not the transformation details.
NTv2 grid shifts are part of datum transformations, which are handled separately in ArcGIS Pro.
NTv2 grids are government-owned datasets and have licensing or distribution restrictions.

Uncorrected GPS data

Using standard/uncorrected GPS has an accuracy of only ~3m, which is less accurate than the correction here. Therefore most users do not need to worry about the level of precision converting to and from the standard GPS International Terrestrial Reference Frame (ITRF) used by GPS is ITRF2020, with WGS84(G2296) being the aligned version used in GPS since January 2024.