Slope Map and Terrain Slope Calculator
Draw an area on the map to get a slope map, mean and maximum slope, and the area in each slope class. You can also measure the slope between two points.
Summary
Click a coloured cell on the map to see the slope and elevation at that spot.
This is the average gradient of the straight line between the two points; bumps and dips in between are not included.
Click point A on the map, then point B, or type the coordinates.
Area by slope class
Class areas are the share of cells inside the polygon applied to the geodesic polygon area. 1 dönüm (Turkish land unit) = 1,000 m², 1 hectare = 10,000 m².
Data Source and Accuracy
Elevation data: AWS Terrain Tiles (Mapzen/Tilezen; SRTM, GMTED2010, ETOPO1 and other sources), roughly 10-30 m resolution. Results are for preliminary assessment and do not replace a field survey. Everything runs in your browser; the area you draw is not sent to any server.
Related Tools
What terrain slope is and why it matters
Slope describes how much the ground rises or falls per unit of horizontal distance. As a percentage it is slope (%) = height difference / horizontal distance × 100; in degrees it is arctan(percent / 100). A hillside that climbs 10 m over 100 m has a 10% slope, which is about 5.7 degrees. A 100% slope equals 45 degrees. Because the two units are not linearly related, it is worth keeping them apart.
Slope is one of the first things to check when screening a plot or a field, planning farming and irrigation, sketching a road, canal or pipeline route, choosing sites for land-hungry facilities such as solar farms, or flagging hillsides prone to erosion. This tool builds a slope map for the area you draw and tells you how much of it falls into each slope class.
How to use it
- Zoom in to the place you want to study. If you know the coordinates, type the latitude and longitude into "Go to location".
- Draw a rectangle or a polygon with the toolbar on the map or the buttons in the panel. Finish a polygon by clicking its first vertex again. Without a mouse, "Analyse Visible Area" analyses what is currently on screen.
- The tool splits the area into a regular grid, reads the elevation of each cell and computes slope. The result is added to the map as a semi-transparent, colour-coded layer whose opacity you can adjust with the slider.
- The summary lists mean, minimum and maximum slope, the lowest and highest elevation and the polygon area. The table gives the share of each slope class in m², dönüm and hectares. Clicking a coloured cell shows the slope and elevation at that spot.
- In "Two-Point Slope" mode, click two points to get the height difference, the horizontal distance and the average slope between them in percent and degrees.
- Download the class distribution as CSV and the slope layer as PNG.
Reading the slope classes
The map uses seven classes: 0-2% (0-1.1°) level, 2-5% (1.1-2.9°) gently sloping, 5-10% (2.9-5.7°) moderately sloping, 10-15% (5.7-8.5°) strongly sloping, 15-25% (8.5-14.0°) steep, 25-35% (14.0-19.3°) very steep and above 35% (over 19.3°) extremely steep. These are general-purpose breaks. Land capability classification, road design or energy projects may use different thresholds, so check the relevant regulations and specifications before making decisions.
Data and method
Elevations are read from AWS Terrain Tiles (Mapzen/Tilezen), which are published as open data. The tiles are used at zoom level 14, about 10 m per pixel at the equator; for Turkey the underlying data comes largely from global models such as SRTM with a resolution of roughly 30 m. Tiles are downloaded straight from the AWS server into your browser and decoded there. The area you draw is never sent to a server.
The bounding box of the drawn area is divided into a grid with a spacing of at least 10 m. The cell count is capped at about 62,500; as the area grows the spacing grows too, and areas over 100 km² are not accepted. Slope for each cell is computed with Horn's method from its 3x3 neighbourhood: the east-west and north-south rates of change are found with weighted differences, giving the directly adjacent neighbours double weight, and slope is the magnitude of those two components. The grid is sampled one cell wider on every side so that edge cells also have neighbours. Class areas are the share of cells inside the polygon applied to the geodesic polygon area.
Icons come from Lucide (ISC licence). Basemaps are provided by Esri and OpenStreetMap contributors. Open-source libraries used and their licences: Data Sources.
Accuracy limits
- Data resolution: The effective resolution of the source model is roughly 10-30 m. Features that change over short distances, such as embankments, cuttings, terraces or stream banks, are smoothed and may look gentler than they are.
- Small plots: A plot of a few thousand square metres covers only a handful of cells. At that scale the result gives a general idea and may not represent the real slope of the plot.
- Built-up and wooded areas: Radar-derived models can be influenced by buildings and tree canopies, so in towns and forests the surface may differ from the bare ground.
- Grid spacing: For large areas the spacing rises above 10 m, which smooths steep slopes further.
- Official survey: This tool does not replace a field survey with a GNSS receiver or a total station. Zoning, design and construction decisions require an official survey by a licensed surveyor.
For professional projects
For how slope is weighed together with aspect, access and grid distance when assessing land for a solar power plant, see the guide how to choose land for a solar plant (Turkish). Projects that need a multi-criteria assessment are covered on the site selection analysis page.
Frequently Asked Questions
How do I convert percent slope to degrees?
Use degrees = arctan(percent / 100). A 10% slope is about 5.7 degrees and a 100% slope is 45 degrees. The relationship is not linear, so a 20% slope is not twice as many degrees as a 10% slope.
Can this tool give me the exact slope of my plot?
No. The tool uses a global elevation model with an effective resolution of roughly 10-30 m and is meant for preliminary assessment. At plot scale, especially for small plots and built-up areas, results can deviate. Zoning, design and construction decisions require an official survey carried out on site with GNSS or a total station.
Why do the two-point slope and the area slope differ?
The two-point slope is the elevation difference between the two ends divided by the horizontal distance, so it ignores any bumps or dips in between. The area slope is computed locally for each cell from its eight neighbours. On rugged hillsides, local slopes can be much steeper than the average gradient between two distant points.
How large an area can I analyse?
The bounding box of the drawn area can be up to 100 km². Grid spacing is 10 m for small areas; for larger areas the cell count is capped, so the spacing grows and the tool shows a notice.