Datum Shift & Projection Distortion Explorer
What is a Datum Shift?
A datum shift is the mathematical transformation applied when converting coordinates between two geodetic datums that use different reference ellipsoids or different origin points. Ignoring datum shifts causes position errors ranging from a few meters to hundreds of meters.
- 3-Parameter (Molodensky): Applies X, Y, Z translations between ellipsoid centers. Accuracy: 5-10 meters. Used for older regional datums.
- 7-Parameter (Helmert / Bursa-Wolf): Applies X, Y, Z translations + 3 rotations + 1 scale factor. Accuracy: 1-2 meters. Standard for modern survey-grade transformations.
- Grid-Based (NTv2): Uses pre-computed shift grids at regular intervals. Accuracy: 0.1-0.01 meters. Used by national geodetic agencies (e.g., Japan GSI for Tokyo Datum to JGD2011).
Web Mercator Scale Factor Calculator
Web Mercator (EPSG:3857) distorts distances and areas. The scale factor increases away from the equator. Enter a latitude to see the local distortion ratio:
Why Datum Shifts Matter in Real Projects
When a Japanese public works survey delivers coordinates in the JGD2011 Plane Rectangular system (e.g., EPSG:6677) and you overlay them on a Google Maps layer (EPSG:3857 based on WGS84), the positional difference between JGD2011 and WGS84 is typically 0-2 cm. However, if the source data uses the older Tokyo Datum, the offset jumps to 400-500 meters without a proper datum shift grid.
FAQ
Q: Is WGS84 the same as JGD2011?
Practically, they are equivalent at the centimeter level. Both are based on the GRS80 ellipsoid. For survey-grade work requiring millimeter precision, they are distinct reference frames with different epoch definitions.
Q: What happens if I skip the datum transformation?
Your GIS features will be offset from their true positions. The error magnitude depends on the datums involved: WGS84 to NAD83 is usually less than 2 meters, but Tokyo Datum to JGD2011 can be 400+ meters.