GIS Tools - CRS Metadata Inspector

What Is a CRS Metadata Inspector?

A Coordinate Reference System (CRS) defines how the numbers in a spatial dataset map to real locations on Earth. Without it, a column of x and y values is meaningless — they could be longitude and latitude in degrees, eastings and northings in meters, or some custom local grid. The CRS Metadata Inspector on gis.tools is a free, browser-based tool that reads a GIS file, extracts every CRS clue it can find, and tells you exactly what projection and datum the data uses, including the EPSG code, the projection name, the underlying datum, the unit of measure, and the full Proj4 or WKT definition string.

For anyone who has ever opened a shapefile and seen features land in the Atlantic Ocean instead of London, the CRS inspector is the diagnostic tool that explains why. It's also indispensable when receiving data from an unfamiliar source, when migrating between CRS-aware databases, or when troubleshooting reprojection bugs.

How CRS Detection Works

Different file formats encode CRS information in different places. The inspector knows where to look in each:

Shapefile — A sibling .prj file containing a WKT string. The inspector parses the WKT and matches it against the EPSG database.
GeoTIFF — Embedded GeoKey tags in the TIFF directory, which encode projection parameters in a structured form.
GeoJSON — The deprecated crs member if present, otherwise assumed WGS84 per RFC 7946.
KML / KMZ — Always WGS84 by definition.
GPX — Always WGS84.
PostGIS / SpatiaLite dumps — Embedded SRID and EWKB tags.
MapInfo TAB — A sibling .tab file containing a CoordSys clause.

For each format, the inspector extracts the raw CRS string, attempts to match it against the EPSG database (the authoritative registry of standard CRS definitions), and reports the result. If no exact match is found, it tries to identify the closest equivalent and warns about ambiguity.

EPSG Matching

The EPSG Geodetic Parameter Dataset is the canonical registry of coordinate reference systems, maintained by the IOGP. It assigns each CRS a numeric code (4326 for WGS84, 3857 for Web Mercator, 32633 for UTM Zone 33N, and so on). The inspector includes a snapshot of the EPSG database compiled into the browser bundle and matches incoming WKT strings against it.

Datum vs Projection

A CRS combines a datum (a specific model of Earth's shape and orientation) with a coordinate system (often a map projection). The inspector reports both: it tells you whether your data is in NAD83 vs NAD27 vs WGS84 (which can differ by tens of meters), and whether the coordinates are projected or geographic.

Practical Applications

Receiving Mystery Data

A contractor sends you a shapefile with no documentation. The inspector tells you the CRS in five seconds, so you know whether to reproject before merging it with your other layers.

Reprojection Debugging

A reprojection step in your pipeline produces wrong results. The inspector confirms whether the source CRS is what you assumed, eliminating one variable in the bug hunt.

Cross-Border Data Integration

Integrating datasets from multiple countries means dealing with multiple national CRSs. The inspector helps you triage which need reprojection.

Historical Data Recovery

Old surveys may use obscure local datums. The inspector identifies them and points to the correct Proj4 transformation parameters.

Open Data Validation

Government open-data portals sometimes mislabel CRS metadata. The inspector lets users catch the discrepancy before building products on top of bad data.

Mining and Surveying

Raw survey data often comes in highly local grids that need to be matched against standard CRSs before downstream processing. The inspector helps surveyors identify which national grid their data uses.

Aviation and Maritime

Nautical and aeronautical charts use specialised CRSs. The inspector recognises common ones and warns about non-standard definitions.

Step-by-Step Workflow in gis.tools

Open the CRS Metadata Inspector page.
Drag your GIS file (or zipped shapefile bundle) into the drop zone.
The inspector parses the file and extracts CRS metadata.
Review the report: EPSG code, projection name, datum, unit of measure, axis order, and full WKT/Proj4 string.
Click Copy Proj4 or Copy WKT if you need the definition for another tool.
If a reprojection is needed, hand off to the EPSG Reprojector & Coordinate Converter.

Worked Example

A planner receives a shapefile labelled parcels_2024.zip from a county GIS department. They drop it into the inspector, which reports the CRS as EPSG:2236 — NAD83 / Florida East (ftUS). The unit is US survey feet, not meters. The planner now knows that any distance or area calculations must account for the unit, and that overlaying with other state-level layers in EPSG:4269 (NAD83 geographic) requires a reprojection. They copy the Proj4 string and use it to transform the parcels into WGS84 for a web map.

Common Pitfalls and Gotchas

Missing .prj file — Without it, shapefile CRS is unknown. Many tools default to WGS84, which is wrong about half the time.
WKT variants — There are several flavours of WKT (WKT1 ESRI, WKT1 GDAL, WKT2 OGC). Tools may match one variant and not another.
Datum confusion — NAD83 and NAD27 differ by tens to hundreds of meters in some areas. Check the datum, not just the projection.
Axis order — Some EPSG entries use lat,lon order; others use lon,lat. The inspector reports the canonical order.
Custom CRS — Locally defined CRSs may not match any EPSG code. The inspector reports the raw string.
Web Mercator confusion — EPSG:3857 (web mercator), EPSG:900913 (Google's old code), and EPSG:3785 (deprecated) are essentially the same but cause confusion in pipelines.
Unit mismatch — Feet vs meters vs degrees can produce wildly wrong distance calculations.

Tips for Best Results

Always include .prj files with shapefile bundles.
Verify the EPSG code matches your expectation before processing further.
Note the unit of measure — it determines how distances and areas scale.
Cross-check ambiguous CRSs by inspecting a known point and confirming it falls where you expect.
For unfamiliar projections, consult the EPSG Reprojector & Coordinate Converter.
Use the inspector after every conversion step in a long pipeline to catch reprojection bugs early.

Comparison with Other GIS Approaches

GDAL's gdalsrsinfo command-line tool is the canonical way to inspect CRS metadata. QGIS displays CRS in the layer properties. PostGIS has Find_SRID and ST_SRID. The browser-based inspector is the most accessible option for non-developers and the only one that requires no installation.

Performance Considerations

CRS detection is fast — a few milliseconds for almost any file. The bottleneck is reading the file itself, not the CRS extraction. The EPSG database snapshot is a few megabytes.

Data Privacy and Browser-Based Processing

The inspector reads only header metadata, not the geometry coordinates themselves, but everything still runs locally in your browser. No data leaves your machine.

Related GIS Concepts

EPSG Code — A numeric identifier for a CRS, assigned by the IOGP's EPSG Geodetic Parameter Dataset.

Proj4 String — A compact text format for describing CRSs, originally from the PROJ library.

WKT (CRS Variant) — Well-Known Text for CRSs, distinct from WKT for geometries; defines projection parameters in a structured XML-like format.

Datum Transformation — The mathematical operation that shifts coordinates from one datum to another, often involving a 7-parameter Helmert transformation.

Frequently Asked Questions

What if my file has no CRS metadata?

The inspector will report this and suggest common defaults based on the geometry's bounding box.

Can I look up an EPSG code without a file?

Yes — the inspector includes an EPSG search box where you can type a code or name.

Why are NAD83 and WGS84 sometimes treated as identical?

In many regions they are within a meter or two of each other, but the difference grows over time as tectonic plates move. For high-precision work, treat them as distinct.

Does the inspector reproject my data?

No — it only reads metadata. Use the EPSG Reprojector & Coordinate Converter for actual transformations.

CRS Metadata Inspector

PRJ / WKT Content

Common CRS Codes