Can Existing Data Support a 1:10,000 Landslide-Risk Map?
A municipality in Georgia needs a landslide-risk map for a 90 km2 area. The only complete topographic map is a 1986 Soviet-era 1:50,000 sheet with 20 m contours. A 30 m resolution satellite DEM is available, along with 120 landslide locations recorded by staff using handheld GPS with typical error of plus or minus 10 m. The planning department wants a 1:10,000 map for a new settlement and road corridor, but the survey budget allows only 20 field days and two surveyors. The old map shows some roads and villages that no longer exist. Aerial photography from 2019 exists but has no georeferencing metadata. The municipality asks whether it can produce a defensible 1:10,000 risk map, how many map sheets would be needed, and what should be checked in the field.
1. Evaluate the suitability of each dataset for a 1:10,000 landslide-risk map.
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The old 1:50,000 map is unsuitable as a base for 1:10,000. A 20 m contour interval is too coarse to show the slope breaks and benches that control landslides; at 1:10,000 the map would imply false precision. The 30 m DEM is better than nothing but its pixel size smooths small scarps and gullies, and its vertical accuracy is unknown. The 120 landslide points are sparse for 90 km2, about 1.3 per km2, and a plus or minus 10 m GPS error is acceptable at map scale but may misclassify a point relative to a scarp. The 2019 aerial photography cannot be used until it is georeferenced with ground control. Therefore the existing data can support a 1:25,000 screening map, not a defensible 1:10,000 engineering map. A 1:10,000 product would require new field survey and higher-resolution terrain data.
2. Calculate the map area at 1:10,000, 1:25,000 and 1:50,000, and recommend a sheet layout.
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Ground area = 90 km2 = 90,000,000 m2. Map area = ground area / scale factor squared. At 1:10,000: 90,000,000 / 100,000,000 = 0.90 m2. At 1:25,000: 90,000,000 / 625,000,000 = 0.144 m2. At 1:50,000: 90,000,000 / 2,500,000,000 = 0.036 m2. A standard A1 sheet is about 0.50 m2, so the 1:10,000 map needs two A1 sheets, or one large custom sheet. The 1:25,000 map fits on about one A2 sheet. My recommendation is a 1:25,000 screening map for the whole municipality, with 1:10,000 inset sheets only for the settlement and road corridor. This concentrates field effort where decisions are most sensitive and avoids pretending that the entire 90 km2 has equal accuracy.
3. Design a 20-day field validation plan for two surveyors. What would you check, and how would you use the results?
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Two surveyors give 40 person-days. Allocate about 5 days to establish ground control points for the 2019 aerial photography, using GNSS at road junctions, bridges and stable rock outcrops. Allocate 10 days to validate a stratified sample of at least 30 mapped landslide points and 20 stable control sites, recording slope angle, soil depth, seepage, cracks, land use and evidence of reactivation. Allocate 3 days to walk the proposed settlement and road corridor, mapping gullies and cut slopes. Reserve 2 days for data entry and a reliability check. Use the results to correct the landslide inventory, estimate positional error, and classify the map as screening or engineering-grade. If more than 20% of sampled points are mislocated or not landslides, downgrade the map and request new terrain data.