Measuring a stockpile once tells you how much is there. Measuring it twice tells you how much has moved — and that's usually the number a site manager, a client or a monthly production report is actually after.
Birdi's Volumetric tool can compare two surveys directly. By setting an earlier survey's DEM as the base surface for a volume on your latest map, Birdi measures the difference between the two terrain surfaces rather than measuring a pile against flat ground. The result is the material that's been added or removed between the two flights.
This article covers:
bringing an earlier survey's DEM into your current map
using the Base DEM method to measure change between two surfaces
reporting the material moved in cubic metres and tonnes
tracking a stockpile across many surveys over time
how to make sure your two surveys are actually comparable
Note: Volumetric comparison requires at least two processed surveys of the same site, each with a DEM.
When to compare two surveys
Monthly production reporting — how much material was extracted, crushed or dispatched this month.
Reconciliation — checking survey volumes against weighbridge, truck-count or plant records.
Earthworks progress claims — how much cut and fill has been completed since the last claim.
Inventory movement — how a stockpile has drawn down or built up between counts.
Rehabilitation and landfill — airspace consumed, or fill placed, over a reporting period.
If you only need to know how much is in a pile right now, you don't need this — a standard volume calculation against the current DEM is what you want. See How to use the volumetric calculator.
Choose your approach
Approach | What you get | Best for |
Base DEM (below) | One figure per area: the material that's moved between the two surveys | Reporting change over a period, earthworks progress, reconciliation |
Two separate measurements | A volume for each survey, differenced yourself in a spreadsheet | Building a time series across many surveys, tracking a pile's balance month by month |
Split Screen and Timeline | A visual before-and-after | Showing someone what changed, not measuring it |
The Base DEM method is the only one that measures the change for you. Start there.
Before you start
Two processed surveys of the same site, each with a DEM (DSM or DTM).
Both surveys ideally flown with the same ground control — see the accuracy note further down. This matters more than anything else in this article.
Decide which surface is "before" and which is "after". You'll be working on the later map and importing the earlier DEM into it.
Step-by-step: comparing two surveys
1. Open your most recent map
Open the later of the two surveys — the "after" capture. This is where you'll do the work, and where the comparison result will live.
2. Bring in the earlier survey's DEM
Click Add Data
Select Add from your workspace
Find the earlier survey — filter by workspace or user profile, or search by Location, Map name or File name
Expand the DEMs folder and select the DEM you want as your base surface
Import it
The imported DEM lands in a new folder named after the map it came from and its original created date, so it's clear which survey it belongs to.
Tip: Open the earlier map in a new tab from the import modal first, to confirm it's the right capture before you bring anything across. Comparing against the wrong survey is an easy mistake to make and a hard one to spot in the result.
Full detail: How to Use "Add Data from Existing Map.
3. Use the same polygon footprint for both surveys
This is the step that makes or breaks the comparison. If you draw a slightly different boundary on each survey, you're measuring two different things and the difference between them is meaningless.
You have two options:
Import the volumetric layer from the earlier survey. In the same Add from your workspace modal, expand the Volumetric layers folder and bring the polygon across. It arrives with exactly the boundary you used last time.
Draw it once and reuse it going forward. From the Add Layer dropdown, select add a volumetric layer, then use the polygon tool to draw your area. Next survey, import this one rather than redrawing.
4. Set your surfaces
Select the polygon in the Layer Panel to open the Volumetric Layer Toolbox, then:
Set the surface layer / Base / Ground DEM to your current survey's DEM — this is the "after" surface
Under Base method, choose Base DEM
Select the imported earlier DEM as the base — this is the "before" surface
Note: Instead of fitting a flat base plane under your polygon the way Linear Fit and Lowest Point do, Base DEM compares your current terrain against the whole earlier surface, point by point. Birdi aligns the two DEMs by spatial position, resolution and extent before comparing, so surveys flown at different resolutions can still be compared.
5. Calculate and read the result
Click Calculate.
Your results describe the volume between the two surfaces:
Cut volume — the volume where your current surface sits above the earlier one
Fill volume — the volume where your current surface sits below the earlier one
Net volume — cut minus fill, the overall material balance across the polygon
In practical terms: if you've been stockpiling, the pile has grown upward and your figure sits in cut. If you've been drawing material down or excavating, the surface has dropped and your figure sits in fill. A net figure near zero means material came and went in roughly equal measure.
6. Report the tonnage moved
Assign a Material in the Volumetric Layer Toolbox and Birdi reports the tonnage alongside the cubic metres — so instead of "3,400 m³ of movement" you can report "3,400 m³ moved, 5,440 t hauled", which is the figure a haulage contractor or a production report is built around.
7. Export your comparison
Open Table view from the Toolbox to see your results in a table, including the Base method and Surface layercolumns — a useful record of exactly which two surveys the figure came from.
From there, Export as a CSV for your own tracking, or as a branded PDF report to send on.
Tracking a stockpile across many surveys
Base DEM gives you the change between two surveys. If you want a running history — a pile's volume month by month — measure each survey independently and keep the results yourself:
On each new survey, use the same imported polygon and measure against that survey's own DEM with Linear Fit or Lowest Point.
Export to CSV from Table view straight after each calculation.
Keep the exports in one spreadsheet, one row per survey date.
Note: Recalculating a volume overwrites its previous result, and Birdi doesn't keep a history of past calculations on a polygon. Export or duplicate the volumetric set after each survey.
Seeing the change on the map
Two views help you show a comparison, though neither calculates anything:
Split Screen — put the two DEMs or orthophotos side by side and drag the central slider to sweep between them. Rasters only.
Timeline — filter your layers and map view by capture date to step through the site's history.
Use these to illustrate a change you've already measured. For the number itself, use Base DEM.
Getting a comparison you can trust
A survey-to-survey comparison is only as good as the alignment between the two surveys. Small differences in georeferencing show up directly in your result, because you're measuring the gap between two surfaces.
Use consistent ground control. The same GCPs across every survey is the single biggest improvement you can make to comparison accuracy.
Expect some variance without it. A difference of up to 10–15 cm between RTK-only surveys is within normal industry tolerance. Spread over a large area, that becomes a meaningful volume — so a small measured change may sit inside your survey noise rather than representing real movement.
Match your surfaces. Compare DTM to DTM, or DSM to DSM. Comparing a DSM against a DTM measures vegetation and structures, not material movement.
Watch for changes that aren't material. Vegetation growth, plant and vehicles parked in the area, and water levels all show up as volume change. Keep your polygon tight to the working area.
Validate the first one. The first time you run a comparison on a site, check it against a truck count or weighbridge figure for the same period. Once you trust the method on that site, you can rely on it.
Best practice tips
Import the polygon, never redraw it. Same footprint every survey, or the comparison doesn't hold.
Name it for the period, not the pile. Pit 2 — Jul to Aug tells you what the number means six months later. Volumetric Layer 4 doesn't.
Work on the later map. Import the earlier DEM into the newer capture, not the other way round.
Export straight after calculating. There's no calculation history to go back to.
Fly the same way each time. Same altitude, overlap and ground control makes every future comparison better.
Keep a Reports folder on each map so your exported comparisons stay findable.