Coordinate Transformation Calculator

Two-Point Coordinate Transformation Calculator

This free coordinate transformation calculator converts coordinates between a local site grid and another plane coordinate grid using a two-point 2D similarity transformation (scale, rotation and translation).

Important: this is not an OSTN15 transformation and does not convert OSGB36 National Grid coordinates to or from latitude/longitude. Two control pairs produce an exact fit, so an independent check point is strongly recommended.

How to use this tool

1. Enter control pointsAdd two known points, entering each point in both the source and target coordinate systems.

2. Add your coordinatesSelect the transformation direction, choose the output precision and enter one or more points to convert.

3. Transform and verifySelect Transform coordinates, review the scale and rotation, then check an independent known point before using the results.

Quick example: select Load test example to populate sample control points and immediately see a verified result.

Good practice

Use control points that are accurate, well separated and cover the area being transformed.
Keep all coordinates in the same linear unit, normally metres.
Use Source → Target for local-to-project conversion, or Target → Source for the reverse calculation.
Do not treat this calculator as an OSGB36/OSTN15 or latitude/longitude converter.
For setting-out or construction work, verify the output against at least one point that was not used as a control point.

1. Enter two control-point pairs

Use well-separated, independently verified points. All coordinates must use the same linear unit.

Point 1 — source E

Point 1 — source N

Point 1 — target E

Point 1 — target N

Point 2 — source E

Point 2 — source N

Point 2 — target E

Point 2 — target N

2. Add coordinates to transform

Direction

Output decimals

3. Results

Enter control points and at least one coordinate.

Two points cannot reveal residual error. Check at least one independent known point before using transformed coordinates for setting-out or construction.

Point ID	Input E	Input N	Output E	Output N

Export results

Download a spreadsheet-ready CSV or open the print dialog to save a clean PDF.

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Company (optional)

Frequently asked questions

What does this coordinate transformation calculator do?

It calculates a two-dimensional similarity transformation between two plane coordinate systems using two matching control points. The calculation applies scale, rotation and translation.

Can I convert a local site grid to project coordinates?

Yes. Enter two control points in both grids, select Source → Target, then add the local-grid coordinates that you want to transform.

Can I reverse the coordinate transformation?

Yes. Select Target → Source to apply the inverse transformation using the same control-point pairs.

Does this calculator convert OSGB36, OSTN15 or latitude and longitude?

No. It transforms between two plane coordinate grids only. It is not an OSTN15 transformation and does not convert Easting/Northing coordinates to latitude/longitude.

How accurate are the results?

The result depends on the quality, separation and compatibility of the control points. Because two control pairs create an exact mathematical fit, always verify the output with at least one independent known point.

Can I export the transformed coordinates?

Yes. Results can be copied to the clipboard or downloaded as a CSV file.

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