Custom Coordinate System Limitations With File Geodatabases In Qgis

What are Custom Coordinate Systems?

A coordinate system in GIS defines how geographic data is represented on a map. It specifies information about the datum, map projections, units of measurement, and other parameters needed to accurately display locations and geometries.

While most GIS software comes with predefined coordinate systems, QGIS allows users to create custom coordinate systems to meet specialized needs. These customized coordinate systems can use custom datums, specialized map projections, or tweaked projection parameters.

Reasons for creating custom coordinate systems include optimizing distortions for a specific geographic area, using legacy systems, or matching the requirements of particular applications or devices. For example, a custom coordinate system could minimize scale and shape distortions over a city or country.

Common Issues with Custom Coordinate Systems

Inaccurate transformations

A key limitation of using custom coordinate systems in QGIS is potential accuracy issues transforming between coordinate systems. QGIS relies on proj library for coordinate system transformations and custom definitions may not translate as precisely.

For example, if a custom coordinate system uses a custom datum or specialized projection parameters, the calculations to convert data from another coordinate system could introduce distortions, inaccuracies, or errors in representing geometric relationships.

This inaccuracy poses issues if data in different coordinate systems needs to be integrated or overlaid precisely. Positional errors or distortions may be introduced by the custom coordinate system definitions.

Compatibility problems

In addition to transformation issues, custom coordinate systems may create compatibility problems with different GIS platforms and spatial analysis functions.

Defining custom ellipsoids, datums, and projections parameters can confuse software functions that expect standard coordinate system definitions. Analysis functions may fail or produce unexpected outputs.

Software aside from QGIS may also not recognize the custom coordinate system definitions from imported datasets. This can make integrating and sharing data with custom systems difficult.

Limited functionality

Using nonstandard coordinate systems can also limit functionality within QGIS itself. Some geoprocessing tools and plugins may not work as intended with data in custom coordinate systems.

For example, terrain analysis tools relying on elevation models may not properly handle vertical datum shifts from custom coordinate systems. Custom map projections could also create issues with measurement tools or buffer creation.

In general, using standard predefined coordinate systems makes it easier to leverage the full functionality of QGIS and plugin capabilities.

Working Around Custom Coordinate System Limitations

Using alternative projection systems

In many cases, the limitations arising from custom coordinate systems can be avoided by using alternative projection systems in QGIS.

For example, rather than creating a custom projection to optimize a specific area, it may be possible to use well-tested pre-defined projections with localized optimization, like New Zealand Map Grid or the Alaska Albers conformal conic projection.

Using alternative projections centralized in QGIS avoids potential accuracy issues from poor custom coordinate system definitions while still improving distortions for a region.

Converting data to standard coordinate systems

Converting datasets in custom coordinate systems to standardized, widely-used coordinate systems is another workaround. This overcomes software interoperability issues and opens up analysis functionality.

For example, data may be originally supplied or captured in a legacy custom coordinate system. By converting copies to a standard projected coordinate system like UTM or a global latitude/longitude system like WGS 84, it enables better integration and analysis.

Converted datasets can be used alongside the original custom coordinate system data if needed. Documentation helps keep things clear.

Employing datum transformations

Custom coordinate systems often use custom datums that shift coordinates to tailor distortions across a region. But rather than defining an entire custom coordinate system, datum transformations can be used instead.

Built-in datum transformation grids and formulas can convert data between many global and national datum systems to improve regional accuracy. This maintains compatibility with projection systems.

Careful use of datum transformations provides an accuracy benefit without issues from creating complete custom coordinate systems. Documentation is essential to track the transformations used in workflows.

Best Practices for Using Custom Coordinate Systems

When creation of a custom coordinate system is justified, following best practices helps minimize issues:

Thoroughly test transformations

Always rigorously test any data transformations to and from the custom coordinate system. Check for positional accuracy issues before using the custom system in production workflows and analysis.

Check compatibility with analysis tools

Verify that key analysis and data management tools perform as expected with data in the custom coordinate system before relying on its use operationally. Check for errors or unexpected outputs.

Clearly document custom parameters

Carefully record all defining parameters, datum conversions, and other specs related to any custom coordinate systems created. This helps communicate special considerations when sharing data with the custom coordinate system.

Following best practices helps make practical use of custom coordinate systems where beneficial, while avoiding major accuracy, compatibility, and functionality pitfalls.

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