What the projection means
The target coordinate is projected at right angles onto the infinite straight line through the two baseline points. The distance along that line becomes chainage, while the perpendicular distance becomes offset.
signed offset = (Point − Start) · left-normal
Worked coordinate projection example
Use a baseline from E 500000.000, N 200000.000 to E 500100.000, N 200000.000 and assign chainage 100.000 to its start. Projecting point E 500040.000, N 200005.000 gives chainage 140.000, offset +5.000 m (left) and foot coordinate E 500040.000, N 200000.000.
The matching forward calculation is available in the Chainage and Offset to Coordinate Calculator. Running a point through both tools is a useful arithmetic check, but it does not validate the chosen project baseline or chainage datum.
Left and right offsets
Positive is left and negative is right when looking from baseline start towards baseline end. The result also gives the exact foot coordinate on the baseline, which can be useful for checks or CAD construction.
Frequently asked questions
Can the perpendicular foot lie outside the baseline?
Yes. The calculator treats the baseline as an infinite line and warns when the foot lies before the start or beyond the end segment.
Why is my offset sign opposite to another system?
Software packages use different conventions. This tool defines positive as left looking from baseline start to end; reverse the baseline or negate the offset when matching a different convention.
Does elevation affect the result?
No. This is a two-dimensional plan calculation using Easting and Northing only.