Measure/Can you tell drainage from Google Maps
Question · method
Can you tell drainage from Google Maps?
Partly, and the honest split is sharper than most people expect. Aerial imagery is a photograph. It carries surfaces, layout and one moment of weather. It does not carry elevation, and drainage is an elevation question.
This page is about where the line falls, because imagery is genuinely useful right up to it — and worthless one step past.
What you can honestly get from the imagery
- Impervious surface. Roofs, drives, patios and paths are obvious from above, and they are where runoff is generated rather than absorbed.
- Where a roof sheds. Ridge lines and roof planes are visible, which tells you roughly where gutters must be concentrating water.
- Neighboring hard surfaces. The drive next door that slopes toward your boundary is right there in the picture.
- Vegetation contrast. Persistently greener or coarser growth in one strip is sometimes a wetter strip. Sometimes it is a different mowing regime.
- Visible standing water on the day of capture, if you are lucky with the date.
All of that is real information and it costs nothing. Use it.
What a photograph cannot give you
A slope. Not in percent, not in degrees, not over the first three meters from a wall. Two lawns can look identical from above while one falls away from the house and the other falls into it. The top-down view carries no height.
An upstream area. The land contributing runoff to one corner of a building is defined by ridges and divides you cannot see in plan, and it very often extends well past the parcel.
A ponding depth. A hollow is invisible in a top-down photo until it has water in it, and by then you are measuring the weather rather than the ground.
A date you chose. Imagery is captured on a schedule that has nothing to do with your question. A dry frame is not evidence of dry ground.
Google Earth Pro does more than the map view — here is what it documents
This is worth stating precisely rather than dismissively, because the desktop product has real measurement tools and Google documents them.
Google's own help page for measuring in Google Earth describes, in Google Earth Pro on desktop, measuring "the distance or area of a shape on the ground", the circumference of a circle, a "3D path" for "the distance between a building and places on the ground", and a "3D polygon" for "the height, width, and area of 3D buildings".
It also documents an elevation profile: draw a path, choose "Show Elevation Profile", and you can "explore the slope, elevation, and distance along a path", with elevation on one axis and distance on the other, and per-point values on mouseover. The page states this is available in Earth Pro rather than the web version.
Source: Google Earth Help, "Measure distances and areas in Google Earth" — support.google.com/earth/answer/148134, retrieved 17 August 2026. competitor facts · sourced, not inferred
So if what you want is the general fall of a lot along a line you draw yourself, that tool exists, it is free, and it will tell you something true. We are not going to pretend otherwise.
What a drainage measurement is doing that a profile is not
A hand-drawn profile is one transect chosen by a person. A drainage measurement is a different kind of object, and the differences are all about being systematic.
| Question | A profile along a drawn line | A per-wall measurement |
|---|---|---|
| Which line? | The one you chose | Every wall, along its own outward normal |
| Slope value | Read off a chart | Measured over 0.75–3.25 m out from every wall, the same way every time |
| Outline error | Not addressed | Footprint displaced 1 m in 16 directions; direction printed only when all 16 agree |
| Upstream area | Not provided | Routed across the whole surface, sampled at 9 points, published as a range |
| Ponding | Not provided | Depressions filled to spill point; depth kept, 3 cm floor, within 20 m |
| Which building | Whatever you clicked | Resolved and labeled with which identity signal answered |
The bit that is not about tools at all
The reason a slope map — anybody's slope map — is not a drainage answer is that shading a surface by steepness is not routing water over it.
Water reads connection, not steepness. On real ground we have measured 282,572 m² of contributing area in Chicago and 201,312 m² in Pittsburgh that a naive reading would send into hollows only millimeters deep, where it would simply stop — against largest surviving channels of 1,034 m² and 33,931 m². A drainage measurement has to get that right before a single drop is routed, and it has to respect buildings, so modeled flow goes around a house rather than through it.
What this is not
None of this is a finding that a property has a drainage problem, a wet basement, or any defect. It is a measurement of ground slope and surface flow. Whether water has ever reached a particular structure was not measured and is not claimed.
A sensible free workflow, if you want one
- Use the imagery for hard surfaces, roof planes and where the neighbors' water is being pointed.
- Use a documented elevation profile tool for the general fall of the lot along a line or two.
- Go outside with a string line and a spirit level for the first three meters off each wall, which is the part that decides it.
- Get a measured surface when you need a number you can put in front of somebody else.
See what a measured surface says about your lot
The free look finds your building, measures the ground around it and shows the modeled flow and the hollows — the things a top-down photograph structurally cannot contain. A full report on any property is $25.
Common questions
Can you tell drainage from Google Maps?
Partly. Aerial imagery shows layout, hard surfaces, vegetation patterns and sometimes visible wet ground on the day it was taken. It does not carry elevation, so you cannot read a slope in percent off a photograph.
Does Google Earth show elevation?
Google's own help documentation describes an elevation profile in Google Earth Pro on desktop: draw a path and choose "Show Elevation Profile" to explore the slope, elevation and distance along it. It also documents measuring distance, area and 3D building dimensions.
Why can't I just look for wet patches in the satellite image?
A dark patch is one moment in one set of weather, and capture dates are not chosen for your question. A dry image is not evidence of dry ground, and a dark patch may be shadow, mulch or a resurfaced drive.
What does a terrain measurement add?
A slope in percent along each wall's own outward normal, the area upstream of each wall, the depth a hollow fills to before it spills, and an uncertainty on each.
Is the imagery useless then?
Not at all. It is the best cheap way to see hard surfaces, downspout discharge points, curb cuts and where a neighboring roof drains. Use it for what it is good at.