When you work with spatial data in ArcGIS Pro, you add datasets as layers to a Map. The map has a defined spatial reference, and this is what ArcGIS Pro uses as it attempts to align your data to the map. It is important to understand how this behaviour works, how to ensure your data in a different datums have correct transformations applied, and what options are available to change default behaviours.
ArcGIS Pro Maps and Default Spatial References
By default, Maps in ArcGIS Pro will use the spatial reference of the first operational layer added. This means the first layer that is not a basemap sets the coordinate system for maps and local scenes. Before any layers are added, the map is soft set to use the basemap coordinate system. For Esri basemaps, this means WGS1984 Web Mercator (auxiliary sphere). Subsequent layers added after the first will not continue to change the map’s chosen spatial reference.
This is a change in behaviour to ArcMap, where the data frame coordinate system would be ‘undefined’ until the first operational layer was added. This often resulted in features and feature classes being created without any spatial reference defined, a significant challenge for spatial data management.
You can use the ‘Choose spatial reference’ option to specify a different default coordinate system (ArcGIS Pro Options > Application, Map and Scene > Spatial reference). Note that the coordinate system setting does not apply to global scenes, because they only use WGS1984 or CGCS2000.
ArcGIS Pro reprojects data on-the-fly that does not match the spatial reference of the map. For non-projected and projected layers sharing the same datum, it is generally able to do this without issue. When you add a layer with a different datum a warning will appear that a transformation is required to align your data sources correctly.
If you do not see this warning it may be a result of:
- Only one transformation exists between the data coordinate system and map coordinate system, and ArcGIS Pro has automatically chosen this option. You can identify and edit which transformations are currently applied under the Map properties. You can also add transformations for a coordinate system not currently used in the map.
- You have previously chosen to ignore this warning. It can be re-enabled in ArcGIS Pro Options > Application, Map and Scene > Spatial Reference > ‘Warn if transformation between geographic coordinate system is required to align data sources correctly’.
If you are working with a mix of datums and do not have transformations applied it may lead to data misalignment.
Working with existing maps
You can readily identify or change the coordinate system currently used by the map by right-clicking the map name in the ArcGIS Pro Contents Pane and going to Coordinate Systems:
To change the spatial reference for your map, select from the XY coordinate systems available. All available coordinate systems are split into geographic or projected coordinate systems. A search tool is available. You can also save any commonly used coordinate systems by right-clicking them and choosing to add to favourites.
Coordinate systems used by layers currently in the map will also be displayed. You can use these to easily match the Map coordinate system used against a layer. It’s also useful to check what coordinate systems are being used and ensure then transformations are applied as necessary:
To see what transformations are being applied you can move to the Transformations tab in the Map Properties Pane. Remember that transformation paths work in both directions, so the ‘GDA1994 to GDA 2020 1’ transformation will align your GDA94 GCS to GDA2020 maps and vice versa. You can also specify the transformation being used between datums that have multiple.
From ArcGIS Pro 2.7 you can now add additional transformations. These may not be currently used in the map, but can be saved for future use and for when the map is shared as a web layer.
Working with spatial data sources
To adhere to organisational standards and to minimise errors while editing or performing analysis, it is good practice to transform the datum in the source data. This step is commonly performed when bringing data into your file or enterprise geodatabases.
You can identify the spatial reference of a layer in your Content or Catalog pane by right-clicking the item > Properties > Source > Spatial Reference.
For data sources with an incorrect or unknown coordinate system defined you can use the Define Projection tool to modify the coordinate system of the layer. When using this tool note take caution and note:
- You must know the correct coordinate system of the dataset before using this tool.
- The tool updates the input data (Most geoprocessing tools create an output. This does not).
- The Define Projection tool only updates the existing coordinate system information. It does not modify the geometry of the input data; it does not project or transform it into a new coordinate system.
You can use the Project tool to project spatial data from one coordinate system to another. The tool will create an output, meaning your original input data will not be modified and you will have two copies of your data. When running the tool, it will interpret the spatial reference of your input data and you can choose the output spatial reference. The Transformation parameter is optional. When transformation is required, no drop-down list will appear on the parameter, and it is left blank. When a transformation is required, a drop-down list will be generated based on the input and output datums, and a default transformation will be selected. You can change the transformation path as desired.
For example, projecting from GDA94 to GDA94 MGA Zone 56 does not require a transformation as the same datum is used for both. GDA94 MGA Zone 56 to GDA2020 MGA Zone 56 would require a transformation to be selected as the underlying GCS is different.
Which transformation do I pick?
These blogs are not intended to make recommendations on coordinate systems or the transformations between them. The appropriateness may differ depending on your data, organisational and workflow needs. Esri Australia GDA2020 service packages offer a strategy and workplan built on a thorough analysis of your requirements.
Some key technical concepts to note at the time of writing:
- The grid-based transformation methods can be very accurate in modelling changes between two geographic coordinate systems.
- The appropriate NTv2 transformation grid to use between GDA2020 and GDA94 differs depending on the jurisdiction and the accuracy and lineage of the dataset being transformed.
- Currently GDA2020 reference systems are defined in relation to GDA94 only. As a result transforming between other coordinate systems, such as WGS 1984 or Vicgrid GDA94, to GDA2020, may require an additional transformation – a ‘composite transformation’. This brings further considerations and complications.
- Composite and grid-based transformation methods are not supported in all parts of the ArcGIS platform. Data may need to be projected using the Project tool prior to use.
Moving to GDA2020 and looking for a strategy built on a thorough analysis of your requirements? Find out more about Esri Australia GDA2020 service packages and book a free consultation here.
- Understanding Spatial References in Australia
- The introduction of GDA2020 to ArcGIS and how to install additional transformations
- Working with coordinate systems and transformations in ArcGIS Pro (current page)
- GDA2020, ArcGIS Online and the Web Mercator Dilemma