crs-and-reprojection

name	crs-and-reprojection
description	Use when working with coordinate reference systems, EPSG codes, datum mismatches, reprojection choices, projected versus geographic CRS, unit confusion, or map alignment problems in geospatial data.

CRS And Reprojection

Use this skill when the core problem is spatial reference correctness.

Goals

• Determine whether the current CRS information is known, missing, or wrong.
• Explain the operational impact of the CRS on measurements, overlays, and map alignment.
• Recommend a safe target CRS for the task.
• Distinguish assigning a CRS from reprojecting data.

Workflow

1. Establish the current state.
   • What CRS is declared?
   • Is it geographic or projected?
   • What units are implied?
2. Check whether the CRS matches the task.
   • Web mapping.
   • Distance or area measurement.
   • Local engineering workflow.
   • Regional or global analysis.
3. Identify the failure mode.
   • Missing CRS metadata.
   • Wrong CRS assignment.
   • Mixed layers in different CRS.
   • Datum mismatch.
   • Coordinates with suspicious magnitude or axis order.
4. Recommend the correction path.
   • Assign the correct source CRS if metadata is missing but known.
   • Reproject only after the source CRS is trustworthy.
   • Choose a target CRS based on the analysis objective, not habit.
5. State validation checks.
   • Overlay known reference layers.
   • Confirm units.
   • Sanity-check extent and feature alignment.

Decision Rules

• Use a local or regional projected CRS for accurate distance and area work.
• Use geographic CRS only when the workflow truly operates in angular coordinates.
• Avoid Web Mercator for precision measurement analysis.
• If the source CRS is uncertain, stop and resolve that before downstream processing.

Output Format

Observed CRS situation:
Why it is a problem or not:
Recommended source CRS action:
Recommended target CRS:
Validation checks:
Risks / assumptions:

Example Triggers

• "These layers do not line up, help me diagnose the projection issue"
• "Which EPSG should I use for area calculations in this region?"
• "Do I assign a CRS or reproject this dataset?"
• "Why are the coordinates huge and the map is offset?"