Of all the layers you can pull out of a drone survey, a slope map is one of the easiest for a client to read. Show a landowner red for steep and green for gentle, and they usually start pointing at problem spots before you finish explaining the legend. It answers a question we hear constantly: how steep is this ground, and where? That's what drone slope analysis is for.
This guide covers what percent slope actually means, why it has to come from a bare-earth model, how to read a slope map for a few common jobs, and where its usefulness ends. If you want the bigger drainage picture (drainage lines, ponding, catchments and flow paths), that lives in our guide to drone drainage and water-flow analysis. This post sticks to the slope layer.
What percent slope means
Percent slope is rise over run, times 100. If the ground climbs 1 foot over 10 feet of horizontal distance, that's a 10 percent slope. Climb 10 feet over 10 feet and you're at 100 percent, which is a 45-degree angle, not a vertical wall.
That last point is where people get tripped up. Percent and degrees are not the same scale, and they don't line up in a straight line. A few reference points help:
- About 1 degree is roughly 1.7 percent.
- About 5.7 degrees is roughly 10 percent.
- About 26.6 degrees is 50 percent.
- 45 degrees is 100 percent.
The conversion is percent = tan(degrees) × 100. You don't need to do that math on site, but you should know which unit a client is thinking in. Contractors, road crews and site planners usually talk in percent. Some equipment manuals and geology references use degrees. When someone says "anything over 15," ask them which one they mean before you color the map.
Why slope has to come from a DTM
A slope map from drone data is calculated cell by cell: the software looks at how much the elevation changes between each cell and its neighbors. That means it inherits every bump in the surface it's built on.
Build it on a digital surface model and every roof edge, truck and tree line turns into a cliff. You'll get bright bands of "steep ground" around a parked excavator and along a fence row of cedars. None of that is the ground. Slope belongs on the bare-earth digital terrain model, where above-ground objects have been filtered out. If the DSM and DTM distinction is new, our post on DSM vs DTM in drone mapping walks through it.
Cell size matters too. A very fine DTM picks up small ruts, tire tracks and spoil lumps as local steep spots. That's real, but it can drown out the broader grade a client cares about. If the map looks like static, look at it zoomed out, or compare it against contours before you draw conclusions.
Drone slope analysis on real jobs
Most slope maps are shown as color classes, from flat to steep. The class breaks you choose shape the story the map tells, so pick them for the job instead of using whatever default appears. Here's how we'd think about a few common requests.
Driveways and access roads
For a new drive or haul road, the question is where the ground stays gentle enough to build on and where it pitches up. A slope map shows the friendliest corridor at a glance, which is often not the straight line between the gate and the pad. It's a planning picture. The allowable grade for a driveway or road comes from local code and the engineer or contractor designing it, not from the drone map.
Building pads and laydown areas
On a pad, you're usually looking for the flattest usable area and how much it tips. A slope map makes it easy to spot a pad that drains one way, a soft dip in the middle, or a corner that falls away faster than expected. That's a good prompt for a cut-fill conversation with the grading contractor.
Mowable ground and equipment limits
Landowners, parks crews and solar site managers often want to know where mowers and utility vehicles can safely run. Equipment manufacturers publish slope limits in their operator manuals, and those vary by machine. Ask the client which limit applies, then set a class break at that number so the map shows "fine" and "not fine" in plain colors. Don't pick a safety threshold for them.
Erosion-prone banks
Steep, bare banks along ditches, ponds and cut slopes are where erosion usually starts. A slope map flags them, and repeat flights let you compare the same bank over time. If the client needs ongoing erosion documentation flown by a local pilot, Find a Drone Pilot has a guide on hiring a drone pilot for erosion monitoring.
Slope and contours work better together
Contours and slope describe the same ground in two ways. Contours tell you elevation: how high a spot is relative to its neighbors. Slope tells you how fast elevation is changing. Tightly packed contour lines and a hot slope class are the same steep bank shown twice.
We like delivering both. Contours give engineers and CAD users the lines they expect, and the slope map gives owners and project managers something they can read without training. Our walkthrough on generating contours from a local elevation model covers the contour side.
What a slope map can't tell you
Being upfront about limits is what keeps a slope map useful instead of misleading.
- It can't see under dense vegetation. A photo-based DTM estimates the ground beneath thick brush, tall crops or canopy. Slope in those areas is calculated from that estimate, so treat it as lower confidence and say so on the deliverable.
- It doesn't decide compliance. A slope map is not a code, accessibility or permit determination. Those calls belong to the designer, the inspector or whoever has authority over the site.
- It doesn't predict stability. Slope shows steepness, not soil strength, groundwater or whether a bank will slide. That's geotechnical work.
- It reflects the day you flew. Active grading sites change quickly. Date every slope map and fly again before relying on it for a new decision.
Making slope maps in SkyView
SkyView Desktop processes your flights on your own Windows PC with no per-map cloud fee (that's our pricing model, not an industry rule), and builds the DSM and DTM locally. From there you can generate contours and run cut-fill volumes.
The slope layer comes from Run water flow analysis, which is part of the Survey & Earthworks add-on (+$99/month, as listed on the features page) and needs a DTM. Run it and SkyView adds Slope, percent, along with Drainage lines, Ponding and Catchments, plus Trace flow path from a click and Outline watershed from a click. The SkyView page describes the workflow.
It shows the shape of the ground: where water wants to go and where it will sit. It doesn't model pipes, storm drains, rainfall, soil or time. Runoff calculations, culvert sizing and grading design belong to an engineer.
Once the maps are done, Pilot Ledger keeps the quote, job record and invoice together, and you can deliver the finished maps to the client through the same account.
If you want to make slope maps from your own flights on your own hardware, see Pilot Ledger plans and pricing.