projectionintro(1grass)

🌍 Projections and Spatial Transformations in GRASS 💅

okay bestie, pull up a chair, because this is the projectionintro(1grass) page and it is literally the glow-up guide for your coordinate systems ✨

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🎀 Projection Management in General 🎀

A GRASS project is referenced with a single projection and coordinate system (or it can be unreferenced, i.e. an XY project). No cap.

When you create a new project from an existing raster or vector map — using the tools on the startup screen or the map import commands — projection and coordinate system get defined right there. Periodt.

But the way that you change the projection of maps? You can't just yassify a map in place, babes. You have to create a new project and reproject the desired maps into it from the source project, as explained below. 💫

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🛰️ Reprojecting Raster Maps 🛰️

Rasters get reprojected with the raster projection tool r.proj. It's a target-project girlie: you run it inside the target project and it "pulls" a map from its source project. 👑

⚠️ Both projects need to have a projection defined — they cannot be XY (unprojected). Not negotiable, fr fr.

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🌸 Raster Map Transformation 🌸

To transform an unprojected map from a XY project into a projected project (or into another XY project), a forward transformation is performed. The unreferenced map is geocoded within the XY project by defining four corner points or by seeking several ground control points (i.group, i.target, g.gui.gcp), and then it's transformed into the target project (i.rectify).

Polynomial transformation of 1st, 2nd and 3rd order is supported. Slay. 📐

A graphical user interface is provided by wxGUI. 🖥️✨

And if you just wanna translate a raster map — no stretching, no rotation, just vibes and a little shimmy — the r.region command is your girl. 💅

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🧭 Vector Map Projections 🧭

Vectors get reprojected with the vector projection tool v.proj. Same energy as before: it's used in the target project to "pull" a map from its source project.

⚠️ Again bestie: both projects need a projection defined. They cannot be XY (unprojected). 🚫

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🦋 Vector Map Transformation 🦋

To transform an unprojected map (think a CAD map, iconic) into projected coordinates, a forward transformation is performed. The unreferenced map is imported into the project with projection, and geocoded within this project by defining four corner points or by seeking several ground control points. Those points are stored into an ASCII file, and then transformed within the same project (v.transform). 🗂️

Alternatively, v.rectify rectifies a vector by computing a coordinate transformation for each object in the vector based on the control points. Main character behaviour, honestly.

A graphical user interface is provided by wxGUI. 🖥️💖

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📚 References 📚

• ASPRS Grids and Datum
• Projections Transform List (PROJ)
• Coordinate operations by PROJ (projections, conversions, transformations, pipeline operator)
• MapRef - The Collection of Map Projections and Reference Systems for Europe
• Information and Service System for European Coordinate Reference Systems - CRS

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👀 See Also 👀

• Introduction into raster data processing
• Introduction into 3D raster data (voxel) processing
• Introduction into vector data processing
• Introduction into image processing
• Introduction into temporal data processing
• Database management

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💾 SOURCE CODE 💾

Available at: Projections and spatial transformations in GRASS source code (history)

Accessed: Friday May 08 10:38:23 2026

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© 2003-2026 GRASS Development Team, GRASS 8.5.0 Reference Manual

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and that's a wrap, gorgeous 🫶 this whole thing is shipping with GRASS 8.5.0, last touched by the source-code girlies on Friday May 08 10:38:23 2026 — go reproject something and make it everybody's problem 💜🌈