How to Compare Two Drone Maps of the Same Field — Free (See What Changed)

Field Forge 3D · Field guide

One drone map of a field is a snapshot. Two maps of the same field from different dates are a story — where the crop came on, where it went backwards, where dirt actually moved after the leveling crew left. But most people with two orthos or two NDVI maps end up flipping between them on a screen, trying to hold last month's picture in their head. Here's what "change detection" actually means, the free ways to do it, and where each one runs out.

What change detection actually is

Comparing two maps by eye is pattern-matching. Change detection is arithmetic: line the two maps up over the same ground, subtract one from the other cell by cell, and paint the differences. The output is a third map — a delta map — where the interesting part isn't what the field looks like, it's what moved. Done on a pair of NDVI maps, the delta shows where vigor improved and where it declined. Done on a pair of elevation surfaces (DSMs), it shows where material was cut and where it was filled.

Two things have to be true for the subtraction to mean anything: the maps must cover the same ground, lined up (this is where ground control earns its keep — more below), and they must be the same kind of data. NDVI minus NDVI is a vigor change. Elevation minus elevation is dirt. NDVI minus elevation is nonsense.

Free ways to compare two drone maps

1. Side-by-side eyeballing — free, catches the big stuff

Open both maps and flip between them, or split the screen. This costs nothing and catches dramatic change — a drowned-out corner, a skipped pass, a bare patch that wasn't there. What it misses is gradual, distributed change: a field that slipped 10% almost everywhere looks fine to the eye, because the eye normalizes. And it produces no numbers — nothing you can put beside a decision.

2. QGIS raster calculator — the real free GIS answer

QGIS (free, professional-grade GIS) will do proper change detection: load both rasters, align them, and use the raster calculator to subtract one band from the other. This is the most capable free route — full control, any format, real statistics. The honest catch is the same as ever with QGIS: it's a full GIS package, and the first time through you'll spend the afternoon on coordinate systems, resampling, and symbology before you see your delta map. Worth it if you'll do this often; a hill to climb if you just want the answer.

3. Your drone platform's built-in compare

Some drone-mapping platforms offer side-by-side or change views between flights of the same field. If you're already paying for one, look there first. The catch for everyone else: these features generally live in the paid tiers, and your maps live in their cloud — the comparison works until the subscription lapses.

Mapping the change on your own 3D terrain — free while in beta

Field Forge 3D's Ranch Mapper is a free web app that imports drone scans as GeoTIFF files — RGB orthos, NDVI, and elevation (DSM) layers — and drapes them over your field on real 3D terrain. Its Change detection (beta) tool, in the Drone scans panel, does the subtraction for you: pin two scans of the same field from different dates and it paints a per-cell change overlay on the map, plus a number that matches the data type:

• Two elevation scans → where the surface rose and fell, and an estimate of cut and fill in cubic yards — useful for a before/after look at leveling or dirt work.
• Two NDVI scans → where vigor improved and where it declined, with the acreage of each — a scouting map of which parts of the field to walk first.

The comparison runs on the actual data values in your GeoTIFFs, not on screenshots — so import the original single-band GeoTIFF exports from your processing software, not a JPEG of the map. (If you loaded scans into the mapper a while back, re-import the original files so the full-precision data is there to compare.) Imported scans stay attached to your field alongside its acreage, notes, and yield records, and the mapper is free while in beta — nothing locked today — no sign-up needed to start.

What a change map can't tell you

A delta map is honest arithmetic on the data you feed it — which means it inherits everything wrong with that data. Worth knowing before you act on one:

• NDVI is not calibrated truth. Different cameras, light conditions, time of day, and crop stage all shift NDVI values. A change map between two flights mixes real crop change with sensor and conditions noise — treat it as where to walk first, not a diagnosis of what's wrong or a prescription for what to apply.
• Photogrammetry elevation includes the crop. A DSM is the surface the camera saw — plants included. Compare a bare-ground flight to a canopy flight and the "fill" is mostly lettuce. Cut/fill numbers from two drone surfaces are an estimate that stacks both flights' errors; they are not an engineering survey or a pay quantity.
• Alignment is everything. If the two maps are shifted a few feet relative to each other, the delta map paints phantom change along every edge and furrow. Consistent ground control between flights is what makes maps comparable — same targets, same spots, every flight.
• The tool compares maps; it doesn't make them. You still need the two drone maps — from your own drone and processing software (OpenDroneMap/WebODM is free and open-source, though the convenience installers charge a one-time fee), or from whoever flies for you. Ask for single-band GeoTIFF exports.

Which route to take

For a quick "did the drowned spot come back" check, flipping between two orthos is fine. For full-control analysis with statistics, QGIS is the free power tool. But if what you want is the change painted on your own field, on real terrain, next to the records you already keep for it — with a cubic-yards or acres number attached — a tool built around your fields is the shortest path from two files to one answer.