A difference grid is a map layer that shows the change in elevation between two surfaces. Birdi subtracts one surface from the other and colours the result, so cut and fill, settlement, growth and material movement read at a glance — no exports, no third-party software.
You can:
Create a difference grid from any elevation surface on your map
Compare against surfaces from any map in your workspace, not just the one you're working in
Compare any two single-band surfaces — including difference grids you've made earlier
Set the colour scheme and value range before you create the layer, and restyle afterwards
Click any pixel to read the exact metres of change at that point
Difference grids are available on Growth and Ultimate plans.
When to use a difference grid
You want to see how much material has been cut or filled between two flights
You're tracking progress on an earthworks or civil construction site
You need to check a stockpile area for movement between surveys
You're monitoring settlement, subsidence or embankment movement over time
You want to spot unexpected change across a whole site, rather than measuring one area you already suspect
You're reporting change to a client or stakeholder and need a clear visual, not a table of numbers
You want a rate of change — compare two difference grids to see whether movement is speeding up or slowing down
Step-by-step: creating a difference grid
1. Open your map and find an elevation surface
Open the map containing one of the two surfaces you want to compare
In the Layers panel, find your DSM, DTM or other elevation surface
Either surface can be your starting point — you'll pick the second one in the next step.
2. Open the layer menu
Right-click the elevation surface in the Layers panel
3. Select Create difference grid
Choose Create difference grid from the menu
The difference grid panel opens, with your selected surface set as one side of the comparison
4. Choose the surface to compare against
Pick the second surface from the list
Surfaces already on your current map are listed first; everything else in your workspace is listed below
Each surface shows its survey date, so it's easy to pick the right two
You can compare any two single-band surfaces — including difference grids you created earlier, which is how you get a rate of change.
5. Set your colour scheme and value range
Choose a colour scheme for the output
Set the value range you want the colours spread across
Tip: A tight value range makes small movement visible; a wide range keeps big cut-and-fill areas readable. You can change both later without regenerating the layer, so don't over-think it here.
6. Name the layer and create it
Birdi suggests a layer name from the two survey dates — edit it if you'd prefer something else
Select Create
Birdi generates the difference grid and adds it to your Layers panel with its own difference grid icon
Larger surveys take longer. You can keep working on your map while it generates, and if a job does fail it's clearly marked as failed rather than sitting in a loading state.
7. Read the change on your map
Open the Details tab to see both source surfaces and their dates
Open the Legend tab to see the colour scale of change
8. Restyle the layer if you need to
Change the colour ramp or value range on the layer at any time
The layer updates without regenerating, so there's no wait
9. Ask Scout instead
If you'd rather not go through the menus, ask Scout, Birdi's AI agent.
Open Scout from the toolbar or the tag on the right of the screen
Ask in plain language — for example "what changed between my June and August surveys?"
Scout offers you a Split Screen comparison or a difference grid, and can open the difference grid panel for you
Best practice tips
Compare like with like. DTM to DTM for ground movement; DSM to DSM when you want stockpiles, vegetation and structures included. Mixing the two will show vegetation as change.
Check the survey dates before you create. The picker shows them for a reason — comparing the wrong two surveys is the easiest mistake to make.
Name layers so the direction is obvious — for example Change – June to August 2026. It saves confusion when several difference grids sit on the same map.
Start wide, then tighten the range. Generate with a broad value range to see the whole picture, then narrow it to bring out subtle movement.
Use difference grids to find change, volumetrics to measure it. The grid shows you where to look; the volumetric calculator gives you a cubic-metre figure for a specific area.
Stack difference grids for trends. Comparing two difference grids shows whether movement is accelerating — useful for embankment and settlement monitoring.