Projections

Projections deals with where data sits on the earth: converting a vector layer from one coordinate reference system to another, and giving a raster the georeferencing it lacks.

Three tools, all running in your browser.

Start here

Reproject Vector

Converts a vector layer to another coordinate reference system, preserving its attributes and its topology.

Parameter	What to set
Input	The vector layer to reproject.
Epsg	The destination EPSG code, as a number. 4326 is WGS 84, 3857 is Web Mercator, 31370 is Belgian Lambert 72, 2154 is Lambert 93 for France.
Output	The reprojected layer.

An EPSG code is the standard identifier of a coordinate system. If you do not know the one you need, epsg.io lists them all with their extents.

The Map Builder holds its layers in WGS 84 for display, so the practical use of this tool is to produce a layer in a projected system for export, for instance to hand data to a colleague working in the national grid.

Orthorectification

Corrects the geometry of a raw satellite or aerial image using an elevation model.

Relief displacement is the problem it solves. On an uncorrected image, a mountain top appears shifted away from the point directly beneath it, because the sensor sees it from an angle. The higher the terrain and the further from the centre of the image, the larger the shift, which can reach tens of metres.

Parameter	What to set
Input raster	The raw image. Its RPC camera metadata must be embedded in the file or present in an adjacent RPB file.
Input dem	An elevation model covering the whole scene.
Output epsg	The coordinate system of the result. Defaults to WGS 84, and a projected system such as a UTM zone is usually the better choice.
Output resolution	The pixel size, in the output units. Defaults to the ground sampling distance of the input.
Resample method	Nearest, bilinear, which is the default, or cubic.
Nodata value	The value assigned where no source pixel was available.
Output	The orthorectified image.

The quality of the result follows the quality of the elevation model. A coarse DEM in steep terrain leaves residual displacement.

Georeference Raster From Control Points

Gives georeferencing to an image that has none: a scanned historical map, an aerial photograph, a plan.

You supply control points, pairs matching a position in the image with a known position on the ground, and an EPSG code. The tool computes the transformation, warps the image and can write a report on the residual error at each point.

Parameter	What to set
Input	The image to georeference.
Control points	The matching pairs of image and ground coordinates.
Epsg	The coordinate system of the ground coordinates.
Resample	Nearest, bilinear, cubic, lanczos, average, min, max, mode or median. Use nearest for a classified image, bilinear or cubic for a photograph or a scan.
Report	An optional report of the residual error per point.
Output	The georeferenced raster.

Spread the control points across the whole image, including the corners, rather than clustering them in the middle. Read the report: a point with a much larger residual than the others is usually a mistake in the pairing, and removing it improves everything.

All the tools

Tool	What it does
Reproject Vector	Converts a vector layer to another EPSG coordinate system, keeping attributes and topology.
Orthorectification	Corrects relief displacement in a raw image using a DEM and its RPC camera model.
Georeference Raster From Control Points	Georeferences an image from control point pairs, with a residual error report.

Notes

Reprojecting a raster. There is no general raster reprojection tool in the panel at present. Orthorectification reprojects as part of its work, and rasters loaded in cloud native formats are handled directly by the map.

Check the extent. A projected coordinate system is valid only over a limited area. Reprojecting Belgian data into Lambert 93, the French system, produces coordinates that compute cleanly and mean nothing.