Calculate Z-factor for slope/terrain analysis when DEM is in geographic degrees (EPSG:4326). Enter latitude or upload a GeoTIFF DEM to auto-suggest
This free tool helps you compute the Z-factor required for slope and terrain analysis when your Digital Elevation Model (DEM) uses geographic coordinates (latitude/longitude in degrees), typically EPSG:4326, while elevation is stored in meters.
Many GIS workflows (ArcGIS, QGIS, GDAL-based tools) need a Z-factor to correct unit differences between horizontal units (degrees) and vertical units (meters). This calculator supports:
- Single point (compute Z-factor from one latitude)
- Area / extent (min/max latitude → mean latitude → Z-factor)
- GeoTIFF DEM upload (read bounding box → suggest mean latitude and Z-factor)
Z-Factor Calculator
Use this when DEM horizontal units are degrees (typically EPSG:4326) and elevation Z is in meters. Formula used: Z = 1 / (111320 × cos(|lat|)).
Upload a georeferenced GeoTIFF DEM. We read only spatial metadata (bounding box) to suggest a Z-factor. Drag & drop is supported.
If your DEM is already in a projected CRS (meters, e.g., UTM), you typically do not need this Z-factor approach.
What is Z-factor?
The Z-factor is a scaling value used to convert vertical units so they match the horizontal units used in slope and aspect calculations. When a DEM is stored in geographic coordinates (degrees), one degree of latitude/longitude represents a different ground distance depending on latitude. A Z-factor approximates the conversion from degrees to meters for your area.
How this calculator computes Z-factor
For a geographic DEM (degrees) with elevation in meters, a common approximation is:
Z = 1 / (111320 × cos(|latitude|))
- 111,320 ≈ average meters per degree (latitude)
- cos(|latitude|) accounts for shrinking distance of longitude degrees toward the poles
- |latitude| uses absolute value because only magnitude matters
This tool computes the Z-factor using: latitude (single point) or mean latitude (area/GeoTIFF extent).
When should you use Z-factor?
- Use Z-factor when your DEM is in EPSG:4326 or any geographic CRS where XY units are degrees.
- If your DEM is projected in meters (e.g., UTM), you usually do not need Z-factor because horizontal and vertical units are already compatible.
Why the tool warns about large areas
Z-factor varies with latitude. If your study area spans a large north–south extent, a single mean latitude can produce a Z-factor that is less representative for the full region.
That’s why the tool warns when the latitude span exceeds your chosen threshold, and suggests splitting the area into latitude slices (each slice has its own mean latitude and Z-factor). This improves consistency for regional studies, long transects, or large countries/continents.
Practical examples
Example 1: Single location
If your point is near latitude 34°, the tool computes the Z-factor for that latitude and you can use it in slope tools that expect a Z-factor value.
Example 2: Study area extent
If your area ranges from 32° to 35°, the tool uses mean latitude: (32 + 35) / 2 = 33.5° to compute the recommended Z-factor.
Example 3: Upload a GeoTIFF DEM
Upload a georeferenced GeoTIFF DEM to automatically read its bounding box and estimate latitude span, mean latitude, and a suggested Z-factor.
Common mistakes (and how to avoid them)
- Using Z-factor on projected DEMs: If your DEM is in meters (UTM/Local projection), Z-factor may distort results. Use Z-factor mainly for degree-based DEMs.
- Mixing feet and meters: This tool assumes elevations are in meters. If your DEM uses feet, convert elevations or apply a unit conversion before slope analysis.
- Very large extents: If your study area spans many degrees of latitude, split into slices or reproject to a metric CRS for more robust terrain analysis.
FAQ
Is this Z-factor formula exact?
It is a widely used approximation for converting degrees to meters for slope calculations in geographic coordinate systems. For best accuracy on large areas, reproject the DEM to a metric CRS or compute per-latitude slices.
Does the tool upload my DEM to a server?
No. The GeoTIFF is processed in your browser to read spatial metadata (bounding box). The file does not need to be sent to a server for Z-factor estimation.
What if my DEM is not EPSG:4326?
If XY units are degrees, the tool still applies. If XY units are meters (projected CRS), you typically do not need Z-factor.
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