What you'll do

Give a SWMM model a coordinate reference system, understand the difference between the project CRS, a layer's CRS and the canvas CRS, decide whether a CRS change should reproject your stored coordinates or only re-render them, and recover when the network doesn't sit where the basemap says it should.

Where to find it

Entry point	What it does
Coordinate Reference System button on the status bar	Shows the canvas CRS (e.g. `EPSG:6595`). Click it to open the CRS picker.
Model → Set Project CRS…	The same picker and the same follow-up prompt.
Model → Simulation Options… → Spatial & CRS	Shows the model layer's CRS and extent. Its Change… and Detect from coordinates buttons are inert in this build.
Layer right-click → Properties… → Source	Per-layer CRS with its own Change… button.
The CRS prompt at project open	Appears when the `.inp` has no usable CRS.

Step-by-step

Three CRSes, and which is which

CRS	Lives on	What it means
Project (model) CRS	The SWMM model layer	The system the coordinates in the `.inp` are actually expressed in. Written back to the file as the `[OPTIONS]` `CRS` key.
Layer CRS	Every other layer — vectors, rasters, basemaps, 2D results	The system that layer's own data is in, read from the file where possible.
Canvas CRS	The map canvas	The system the map is drawn in. Everything else is reprojected into it on the fly.

SWMMVis treats the project CRS and the SWMM model layer's CRS as a single thing: whichever route you use to change one — the picker, the Layer Properties dialog, a project restore — drags the canvas along with it, so the two normally agree.

A fresh canvas starts in EPSG:3857 (Web Mercator), which is what OSM and CartoDB tiles are cut in. When a model with a projected CRS loads, the canvas adopts that CRS. When the model's CRS is geographic (latitude/longitude), the canvas deliberately does not adopt it — drawing degrees directly gives a Plate Carrée projection that squashes the map north–south by cos(latitude), so the canvas stays projected and the model layer is reprojected into it.

Opening a model with no CRS

If the .inp carries no CRS, the engine falls back to an unnamed local system and SWMMVis stops to ask, because on-the-fly reprojection of basemaps and GIS layers cannot work without one. You get the CRS picker; cancelling it brings up:

Button	What happens
Choose CRS…	Back to the picker.
Use local projected (ft) / (m)	Assigns a local CRS whose linear unit matches the model's flow units. Coordinates keep their original numbers, the scale bar and measurements get a real unit, and 2D mesh generation has a linear unit to honour — but no basemap can be aligned to it.
Abort Open	Cancels the open; the project window closes.

A model that already carries an auto-generated local CRS (Local (ft) / Local (m)) is not prompted — its units are already known.

FIGURE PLACEHOLDERdocs/manual/images/06_crs_required_prompt.png

The CRS Required prompt with Choose CRS; Use local projected and Abort Open

The CRS picker

Select Coordinate Reference System browses the GDAL/PROJ CRS database.

Control	What it does
Search	Free-text filter over CRS names and codes — type a name or an EPSG number.
Authority	`All`, or a single authority (EPSG, ESRI, …) contributed by the CRS database.
Type	`All`, `Geographic 2D`, `Geographic 3D`, `Projected`, `Compound`, `Geocentric`.
Tree	Grouped Type → Area of use → CRS, with the `Authority:Code` in the second column. State planes group by state, UTM zones by region.
WKT preview	The full WKT definition of whatever is selected.

Above the database entries sits a synthetic Local (no transform) group with two leaves — Local — metres (no transform) and Local — feet (no transform). These mint a CRS that carries a real linear unit but performs no transformation, for data whose units are known but whose georeference is not. The group is hidden when an authority filter or a mismatching type filter is active.

There is no recent-CRS list and no free-form WKT/PROJ entry box in this release: pick from the database or use one of the two Local entries.

The CRS selection dialog with a search term; the type filter and the WKT preview

Changing the CRS of a loaded model

Picking a different CRS while a model with geometry is open opens Change Coordinate Reference System, which states the From and To authorities and offers these choices:

Choice	What it does	When to pick it
Assign CRS without changing coordinates	Declares what CRS the existing numbers already use. The model and canvas adopt it, the geometry cache is rebuilt, and the project is marked dirty.	An unreferenced dataset whose coordinates are already in the chosen CRS. An empty project uses this automatically.
Reproject stored coordinates	Transforms every node coordinate, link vertex and subcatchment polygon vertex through OGR from the old CRS to the new one, and writes the new CRS to the engine. The layer and the canvas both adopt it, the geometry cache is rebuilt, the view refits, and the project is marked dirty.	The new CRS is the correct one for the data and you genuinely want the model converted into it.
Re-render only (display in new CRS)	Stored coordinates are untouched. Only the canvas CRS changes; the layer is reprojected for display. Nothing is written to the model.	The stored coordinates are correct and you only want a different view — e.g. Web Mercator to line up with online tiles.

Cancel leaves everything as it was. When the source CRS is Local, only Assign CRS is available: no geographic transform can be built from an unreferenced local grid. Assign a known source CRS first if the existing coordinates already use it; then choose a target CRS and reproject.

Reprojection is best-effort and batched: a single object that fails to transform is logged and skipped, and the rest still get done. The Message Logs panel reports the counts — nodes, link vertices, polygon vertices.

FIGURE PLACEHOLDERdocs/manual/images/06_crs_change_dialog.png

The CRS change dialog offering Assign CRS; Reproject stored coordinates or Re-render only

What gets written to the <tt>.inp</tt>

Committing a CRS sets the model's CRS string, which is saved in [OPTIONS]:

[OPTIONS]

CRS EPSG:6595

The value is the authority code when the CRS has one, and the full WKT otherwise. The engine stores it as metadata and preserves it in hot-start files for consistency checking; it never uses it to transform anything itself. All coordinate transformation happens in the GUI.

Reprojection changes the in-memory model immediately, but the file on disk only changes when you Save.

Setting a CRS from Simulation Options

Model → Simulation Options… → Spatial & CRS shows the model layer's CRS and its extent.

Control	What it does
Layer CRS	Read-only label; `(local)` or `(none)` when unset.
Change…	Would open the CRS picker and assign the result to the layer — inert in this build; the button is never connected to anything.
Detect from coordinates	Would inspect the model extent and suggest EPSG:4326 when it fits inside geographic bounds — inert in this build for the same reason.

So the page is a read-only summary today: use the status-bar CRS button or Model → Set Project CRS… to actually change the CRS. The page also carries its own reminder that changing the CRS there would update the stored CRS only; the status-bar button is what transforms stored coordinates. See 18 — Simulation Options.

Per-layer CRS and on-the-fly reprojection

Every layer carries its own CRS and is reprojected into the canvas CRS as it is drawn — vectors and annotations by transforming their geometry, rasters and tiled services by warping into the canvas grid. When a layer's CRS already matches the canvas the transform is skipped entirely, so a project whose layers all share one CRS pays nothing for the machinery.

A layer's CRS is visible and editable in Properties… → Source (see 07 — Layers and Data Sources). A vector file that declares no CRS is assumed to already be in the project CRS, and a warning naming the file is written to the Message Logs — this is the usual reason a layer lands somewhere unexpected.

Units, scale and measurement

The canvas CRS determines what a map unit means, and therefore:

the Map Scale readout, which converts pixels to ground distance using the CRS's own linear-unit factor (or a cosine-latitude approximation for a geographic CRS);
the on-canvas scale bar;
the Measure tool, which reports true geodesic distances and areas for a geographic CRS and unit-converted Euclidean values for a projected one — and raw unlabelled numbers for a Local CRS.

See 05 — The Map Canvas.

2D meshes and 2D results

The 2D solver works in SI metres regardless of the model's flow units, so 2D data has to be scaled back before it can be drawn with the rest of the model.

The 2D mesh layer reads the .2dm (or the inline [2D_*] sections) in model units and reprojects from the model CRS to the canvas CRS, like any other vector layer. The mesh file's ;; UNITS: header records whether its coordinates are SI.
The 2D results layer reads coordinates in metres from the HDF5 file, then converts metres → model linear unit, reprojects model CRS → canvas CRS, and applies the scene's Y-flip, in that order.

The metres-per-model-unit factor comes from the results file itself when the file declares one (engine 6.0 and later write a /crs variable), which makes the round trip exact — including the engine's use of the international foot (0.3048) for a CRS whose own unit is the US survey foot. For an older file, or for the live in-process results source, the factor is derived the same way the engine derives it: US-customary flow units and a mesh that is not tagged ;; UNITS: SI (m) gives 0.3048; anything else gives 1.0. A single warning naming the layer is logged when the factor had to be inferred.

The practical consequence: if 2D results appear shrunk toward the CRS origin while the mesh and the 1D network draw correctly, the results file predates the self-describing /crs variable and the mesh's units header is being read instead. Re-exporting the results with a current engine fixes it.

FIGURE PLACEHOLDERdocs/manual/images/06_2d_results_alignment.png

A 2D results layer correctly aligned over the mesh and the 1D network

Recovering from a misaligned basemap

The commonest cause of a network that "doesn't sit on" a basemap is a wrong CRS — usually a neighbouring UTM zone or state-plane zone. Symptoms:

the network is the right shape but offset by hundreds of metres;
the network is mirrored, scaled, or rotated relative to the basemap;
the network is a tiny dot near the origin (a units mismatch: feet read as metres, or vice versa).

The fix is to open the CRS picker, choose the correct CRS for the data, and pick Assign CRS without changing coordinates — your stored coordinates were fine, only their interpretation was wrong. If you don't know which CRS is correct, try the plausible ones for the region; the one that lines up is the right one.

Choose Reproject only when the old CRS was correct and you want the model converted into a new one.

Tips and gotchas

Reproject is destructive to the in-memory model but not to the file until you Save. Quit without saving and the .inp on disk is unchanged.
Re-render is free. It writes nothing and changes nothing in the model, so you can flip between CRSes to compare views.
The status-bar button always shows the canvas CRS. After a re-render the canvas and the model CRS differ on purpose; after a reproject they match.
A geographic model keeps a projected canvas. Don't be surprised that the button reads EPSG:3857 after loading an EPSG:4326 model — that is deliberate and keeps the aspect ratio honest.
A Local CRS blocks basemaps. There is no transform to build, so tiles cannot be placed. Local is for getting measurements and mesh generation working on a model whose georeference is genuinely unknown.
A vector file with no .prj is assumed to be in the project CRS. Watch the Message Logs for the warning when a newly added layer lands oddly.
Removing or reassigning a CRS is not on the undo stack. A reproject can be undone only by reprojecting back, or by closing without saving.

05 — The Map Canvas — the status bar, map scale and the Measure tool
07 — Layers and Data Sources — per-layer CRS and adding basemaps
03 — Working with Projects — the dirty marker and what a save writes
18 — Simulation Options — the Spatial & CRS page
19 — 2D Overland Flow: Meshes, Cell Parameters, Boundary Conditions and 1D–2D Coupling — mesh units and the ;; UNITS: header
21 — Results Layers, Result Styling and Animation — loading 2D results
A5 — Troubleshooting — misplaced layers and alignment problems