Data and method behind the slope angle map

Source data and published grid

Coverage of Finland is based on the Korkeusmalli 2 m elevation model from Maanmittauslaitos (National Land Survey of Finland). It is processed in its original coordinate system, ETRS-TM35FIN, and published on a 2 m grid.

Coverage of Sweden is based on the 1 m Markhöjdmodell DTM COG from Lantmäteriet. Coverage of Norway is based on DTM 1 from Kartverket. For these two countries, the elevations are first averaged into 2 m cells aligned with the original terrain grid. The slope is computed only after this step. The working coordinate system is SWEREF 99 TM for Sweden and ETRS89 / UTM zone 33N for Norway.

All three slope mosaics are published on a 2 m grid. A real gap in the high-resolution source stays transparent; it is not filled with values from a coarser elevation model. Coverage of Norway means mainland Norway, not Svalbard or Jan Mayen.

• Maanmittauslaitos: Korkeusmalli 2 m (in Finnish)
• Lantmäteriet: Markhöjdmodell DTM COG, machine-readable STAC description
• Kartverket: DTM 1 and terrain data

How is the slope angle computed and stored?

The map shows six slope angle classes on a 2 m grid. The colours start at 20°.

• 20–24° blue
• 25–29° light green
• 30–34° yellow
• 35–39° orange
• 40–44° red
• ≥45° magenta

The slope is derived from the height differences between neighbouring elevation cells using Horn's method and is given in degrees. A flat surface is 0°, and 45° means that the height changes by the same amount as the horizontal distance. The Slope calculator (in Finnish) shows how degrees relate to percent.

Storage mirrors the earlier production generator: raw values below 15° are dropped, and the other values are rounded down into 2° classes with the formula floor(slope / 2) × 2. So a slope of 15.0–15.9° is stored as 14. Valid raster values are the even numbers from 14 to 90; the value 255 means either terrain below 15° or missing source data.

The stored values 14, 16 and 18 stay transparent, because the visible colours start at 20°. The map is display-only by design and does not answer point queries for slope angle.

Class limits are approximate. Values are stored rounded down to even degrees, and the map style changes colour at 25, 30, 35, 40 and 45 degrees. So a raw value of 25.0–25.9° shows in the 20–24° class, 35.0–35.9° in the 30–34° class and 45.0–45.9° in the 40–44° class. The 30° and 40° limits fall on even stored values, so they do not shift.

The colours show at every zoom level. When you zoom out, each cell of an overview level uses the highest valid slope angle in the area it covers. A small steep spot is therefore not lost in an average, but steep classes look larger when zoomed out than at close zoom.

Grid size is not the same as certainty

The 2 m grid size is the cell spacing of the published raster. A single value still does not describe the exact surface of a rock, a cliff, snow or vegetation. The vertical accuracy of the Finnish data varies between its quality classes, and the collection date and local quality of the Swedish and Norwegian sources vary from area to area.

Source edges, missing cells, the averaging of 1 m data and the joining of separate surveys can show as local anomalies. The map server reprojects the image to Web Mercator for the browser map with nearest-neighbour resampling, so that the stored classes are not blended together.

Update history

7 October 2026: Class limits clarified; the even stored values move the 25°, 35° and 45° limits up by one degree, to 26°, 36° and 46°. The published data release is 2026-07-30-nordic2 in all three countries, and the slope classes show at every zoom level.

28 July 2026: Documented the 2° storage classes, the 15° raw-value threshold and the display-only map layer.

27 July 2026: Documented the reproducible 2 m data pipeline and the source descriptions for Finland, Sweden and mainland Norway.

15 July 2026: Corrected the grid spacing of the published Norwegian raster to 2 m.