USGS: Coordinate Reference Systems
Official explanation of geographic and projected CRS.
Learn more: USGS: Coordinate Reference SystemsGIS alignment guide
Why satellite images don’t align with your map
Misalignment usually comes from coordinate systems, georeferencing, resolution or sensor geometry—not from a single mysterious GIS failure.
Common causes
- Layers use different or incorrectly assigned coordinate reference systems.
- A raster lacks valid georeferencing.
- CAD data uses local coordinates without a defined transformation.
- Sensor geometry or terrain requires orthorectification.
- Datasets have different resolution, acquisition dates or positional accuracy.
Quick checks
- Inspect each layer’s declared CRS.
- Confirm whether a CRS was assigned or the data was truly reprojected.
- Check coordinates against a known reference point.
- Review raster metadata and ground control.
- Measure the offset before applying a manual shift.
Why it matters
A visually small offset can change parcel attribution, risk overlays, measurements and change-detection results. Alignment must be validated before interpretation.
Assignment is not reprojection
Assigning a CRS labels existing coordinates; reprojecting transforms them. Geographic coordinates use angular units, while projected systems use linear units. Datum transformations, missing georeferencing, orthorectification and terrain can all affect position.
Check before shifting
- Compare declared CRS, datum and axis order
- Inspect raster geotransform, ground control and pixel size
- Check CAD local coordinates and survey control
- Compare acquisition dates and positional accuracy
- Measure the offset against known references before applying any shift
Why small offsets matter
A small visual displacement can change parcel attribution, flood-risk overlays, measurements and change detection. The FEMA workflow demonstrates why GIS/CAD interoperability must be validated before an engineering exhibit is released.
Sources and further reading
PROJ Documentation
Reference documentation for coordinate transformations.
Learn more: PROJ Documentation