How to create contour lines with SRTM DEM and field checks
Contour lines help read terrain, but they are only as reliable as the elevation model behind them.
Field takeaways
- SRTM is useful for reconnaissance and preliminary analysis, not precise elevation delivery.
- Contour interval should match terrain relief.
- Contours should not be the sole source for legal or construction elevation decisions.
- Field checks are needed around drainage, slopes, roads and structures.
- MapLab Survey combines contours, profiles and field notes.
What SRTM provides
SRTM is radar-based global elevation data, and for reading landform over wide areas it is genuinely useful. NASA's Earthdata documentation puts the SRTM 3 arc-second product in the roughly 90-metre resolution class. That makes it valuable for reconnaissance and general slope interpretation — and no substitute for site levels or formal levelling.
Choosing the contour interval
On a flat plain a one-metre interval may draw few lines or simply amplify data noise; in mountains a ten-metre interval may stay too general. For scouting, 5, 10 or 20 metres are sensible starting points, and a finer model over a small area can justify less. But shrinking the interval does not make the data more accurate — it only draws the same uncertainty with more lines.
Surface, not ground
Radar and similar datasets often record the top of vegetation and buildings rather than bare earth, so wooded areas read higher than they are. A grid this coarse cannot represent a bund, a ditch or a terrace.
Using it in the field
Open the contours over a parcel, a route or a working area in MapLab Survey, then walk the critical lines: stream crossings, slope crests, terrace ends, road connections, building surroundings. Where the map says one thing and the ground says another, take a note — fill, cuts, erosion and recent works are usually missing from the elevation data.
The limit on precise work
Contours from global data are a reading aid that speeds up decisions. Channel inverts, foundation levels, road profiles, flood limits and official design levels need control measurements. The surveyor's job is not only to produce the line, but to say how far the line can be trusted.
See it in action
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MapLab Survey handles RTK/GNSS point capture, drawing, area and volume calculations and coordinate conversion, then exports to DXF, KML/KMZ, GeoJSON, Shapefile, GeoPackage, NCZ or CSV. Capture, calculation and export work offline. Live NTRIP corrections and online basemaps need a connection.
Frequently asked questions
Are SRTM contours precise?
They are useful for preliminary analysis, not precise construction elevation.
Which contour interval should I use?
It depends on terrain relief and purpose.
Can contours show drainage direction?
Generally yes, but drainage design needs field elevation checks.
Can MapLab Survey create contours?
Yes, for field interpretation and planning.
Technical references
The field guidance in this article is aligned with the technical and official references below.
Related: Creating an elevation profile · Terrain slope · Cut-and-fill calculation · MapLab Survey