Troubleshooting Projection and Coordinate System Issues

Incorrect projections can cause a patchwork of problems. Projection and coordinate system problems are among the most common challenges in groundwater modeling workflows. Misaligned shapefiles, incorrectly positioned rasters, or models appearing thousands of miles from the expected location can all stem from projection mismatches. Fortunately, the Groundwater Modeling System (GMS) includes tools to help identify and resolve these issues quickly.

This guide covers common projection problems in GMS and highlights best practices for troubleshooting coordinate system issues.

Common Signs of Projection Problems

Projection issues often appear in several ways:

• GIS layers do not line up correctly

• Raster data appears shifted or rotated

• Wells or boundary conditions import into the wrong location

• Model coordinates display unexpectedly large or small values

• Data layers disappear from view after import

These problems usually occur because datasets use different coordinate systems or because projection information is missing.

Step 1: Check the Project Projection

In GMS, begin by reviewing the project’s horizontal coordinate system using:

Display | Projection or the Horizontal Projection dialog.

Verify:

• Datum (e.g., NAD83, WGS84)

• Projection type (UTM, State Plane, Geographic)

• Units (meters vs feet)

Even if two datasets appear similar, small datum differences can cause significant spatial offsets.

Step 2: Confirm Projection Information for Imported Files

Many GIS files rely on accompanying projection definition files such as .PRJ files. If a shapefile or raster lacks a valid projection definition, GMS may assume the wrong coordinate system.

A common best practice is to:

• Verify the source projection before import

• Avoid manually assigning projections unless you know the original coordinate system

• Use consistent units across all datasets

Step 3: Use the New Export PRJ Button

A useful enhancement in newer versions of GMS is the Export PRJ button in the Horizontal Projection dialog. This feature allows users to export the project’s coordinate system information directly into a PRJ file.

This is especially helpful when:

• Sharing data with GIS teams

• Creating shapefiles outside GMS

• Ensuring projection consistency between software packages

Exporting a PRJ file reduces ambiguity and helps maintain alignment between groundwater models and GIS datasets.

Step 4: Reproject Data When Necessary

If datasets use different coordinate systems, reproject them into a common projection before modeling. While GMS can handle many projection conversions internally, using consistent projections simplifies workflows and reduces errors.

For regional groundwater models, projected coordinate systems such as UTM or State Plane are often preferred over geographic coordinates because they preserve distance measurements more accurately.

Final Thoughts

Troubleshooting coordinate system issues in GMS starts with understanding projections, datums, and units. By verifying dataset metadata, maintaining consistent projections, and using tools like the new Export PRJ button, you can avoid many common alignment problems and build more reliable groundwater models.

Make use of the projection capabilities of GMS today!