Geospatial & Environmental

Which geospatial tool for which job

Nine calculators sit here, and they split into three jobs that barely talk to each other. One group traces where water goes on a site. Another sizes the land a renewable project needs. The last measures the ground itself. Get the job straight first, because the tool that tells you how much runoff a storm sheds has nothing to say about how far apart to space wind turbines.

The drainage chain, in order. Site hydrology runs downhill in a fixed sequence, and there’s a tool for each step. Start at the top with the Watershed / Catchment Area Calculator, which delineates the contributing area draining to a pour point, the culvert or inlet you care about. That area is the input to everything below it. Feed it into the Stormwater Runoff Volume Estimator, which turns the drainage area, a design rainfall depth, and a runoff coefficient into the event volume the site sheds. Carry that volume into the Retention Pond Size Estimator to size the wet pond that has to hold it. And run the Basic Erosion Risk Index on the exposed slopes inside the catchment, since bare graded ground is where the sediment that silts up your pond comes from. Miss a step and the error compounds: a pour point placed one DEM cell off throws the area off, which throws the runoff off, which undersizes the pond. The Rainwater Harvesting Tank Size Calculator is the rooftop branch of the same physics, catching roof runoff for reuse before it ever reaches the storm system.

Siting a renewable project. Three tools answer land questions for energy, and they don’t chain the way the drainage tools do. You pick the one that matches the technology. The Solar Land Requirement Calculator converts a system size in megawatts into gross acres, folding in ground coverage ratio and the DC-versus-AC distinction that trips up most first estimates. The Wind Turbine Spacing Calculator works in rotor diameters, turning a turbine’s blade sweep into row spacing and a project footprint. The Off-Grid Solar & Battery Size Estimator is the small-scale cousin, sizing panels and a battery bank from a cabin’s daily kilowatt-hours rather than laying out a utility field. Compare solar against wind on the same parcel, or check whether an off-grid load even fits the roof, and you’re moving between these three.

Measuring the ground. Under both of the other groups sits one question: how big is this piece of land, really. The Contour Area Calculator answers it for sloped terrain, where the flat map footprint (planimetric area) and the actual surface you walk diverge by more than people expect, 6 percent at a 20 degree slope and 41 percent at 45. Use it when a hillside parcel’s usable acreage matters, whether you’re checking a solar site or the drainage area feeding a pond. For a boundary you walked with GPS and only want the enclosed area, the GPS Coordinate Area Calculator is the more direct route.

How these tools compute (methodology)

None of these is a black box. Runoff uses the Rational method, rainfall depth times drainage area times a runoff coefficient, a screening approach that holds up under about 200 acres before travel time across the basin breaks its uniform-intensity assumption. Above that you move to the NRCS TR-55 curve number method. Design rainfall should come from NOAA Atlas 14 point precipitation for your exact coordinates, not a textbook 2-inch storm, because the values shift within a few miles. Runoff coefficients and soil groups follow USDA NRCS conventions, and guessing the soil group can swing the volume 40 percent.

Watershed delineation traces flow direction and accumulation across a digital elevation model, following the divides uphill from the pour point. Resolution drives accuracy: a 10-meter DEM suits rural basins, while an urban site where a graded pad redirects flow wants 1-to-3-meter lidar. Cross-check any delineation against USGS StreamStats for the same point, and if the two areas diverge by more than 15 to 20 percent, the pour point or the DEM is the problem. The renewable tools lean on NREL benchmarks: acres per MW by ground coverage ratio and DC/AC ratio for solar, and the rotor-diameter spacing convention for wind, where 7 to 10 D downwind and 3 to 5 D crosswind manage wake losses. Pond depth, forebay, and freeboard track the EPA National Menu of Stormwater BMPs and state manuals. Every computed value is exact on your inputs. Real-world accuracy rides on the survey and the rainfall data behind them, which is why each page sends you to a licensed engineer before anything gets built.

Terms that trip people up

Runoff coefficient

The fraction of rainfall that runs off instead of soaking in, from 0 to 1. Pavement and roofs sit at 0.85 to 0.95, lawn on sandy soil at 0.10 to 0.20. Area-weight it across a mixed site.

Pour point

The single outlet where a watershed’s flow converges and leaves. Snap it to the DEM flow path, because one cell off clips a side valley or grabs a neighboring ridge.

Ground coverage ratio (GCR)

The share of a solar array’s footprint that’s actually panel, typically 0.3 to 0.4 fixed-tilt and 0.4 to 0.5 for trackers. Array land equals panel area divided by GCR.

Planimetric vs surface area

Planimetric is the flat, straight-down footprint that deeds and GIS use. Surface area is that footprint stretched over the slope, always larger, and equals planimetric divided by the cosine of the slope angle.

Detention vs retention

A retention pond keeps a permanent pool and is sized by volume. A detention basin sits mostly dry and is sized from the gap between inflow and outflow rate. “Capture the first inch” is retention; “detain and release in 72 hours” is detention.