TL;DR: Each method watches deformation at a different scale. Slope radar watches a wall continuously at sub-millimeter precision where people are at risk. Prisms measure fixed points, by automated total station or at each survey. InSAR measures wide areas from satellites every few days. Drone surveys capture the whole surface at each campaign. Most monitoring plans combine them.
Key takeaways
- Radar is the real-time layer. According to Orica, its GroundProbe slope stability radars deliver real-time wall stability data with sub-millimeter precision.
- Satellites see wide areas, but not continuously. According to the European Space Agency, a single Sentinel-1 satellite repeats its orbit every 12 days, and a pair every 6 days.
- InSAR reaches millimeter precision over large areas. According to Copernicus, its Sentinel-1 ground motion service measures movement with millimeter precision and is used to monitor dams.
- Drone-read prisms are checkpoints. According to the help center of Birdi (birdi.io), prisms read from low flights reach about 3 to 4 mm of tagging accuracy.
- The standard does not pick a technology. GISTM leaves the method and frequency to the engineer of record's monitoring design.
What are the main ways to monitor dam and wall deformation?
The four main ways are survey prisms, slope radar, satellite InSAR and drone surveys. Prisms measure movement at fixed points. Radar scans a face continuously from the ground. InSAR measures ground movement over wide areas from repeat satellite passes. Drone surveys capture the whole surface at each campaign. They differ in precision, frequency, coverage and cost.
Prisms can be read in two ways. An automated total station reads them around the clock. Otherwise, a surveyor or a drone survey reads them at each campaign. So prism monitoring can be real-time or periodic, depending on the set-up. The other three methods each have a natural rhythm: radar is continuous, InSAR follows the satellite, and drones follow the survey schedule.
What each method actually measures
Radar and InSAR both measure movement along their line of sight, not full 3D motion. Prisms give 3D movement at each point, horizontal and vertical. Drone surveys give elevation change across the whole surface, plus prism readings where prisms are present. That is why plans combine them: each fills a gap the others leave.
How do the four methods compare?
The four methods compare on precision, how often they measure, how much area they cover, whether they need line of sight, how weather affects them, and cost. Radar leads on precision and speed for a single face. InSAR leads on area. Prisms lead on 3D point movement. Drones lead on whole-surface detail between readings.
| Survey prisms | Drone surveys | Slope radar | Satellite InSAR | |
|---|---|---|---|---|
| Measures | 3D movement of fixed points | Elevation change across the surface | Line-of-sight movement of a face | Line-of-sight movement over wide areas |
| How often | Continuous (automated) or each survey | Each survey, often monthly | Continuous | Each satellite pass, every few days |
| Coverage | Points only | Whole site | One face or pit | Tens of kilometers |
| Precision | Millimeters | Centimeters for surfaces; millimeters for prisms on low flights | Sub-millimeter | Millimeters |
| Best for | Embankment and wall points | Trends, inspection, the visual record | Walls where people are at risk | Regional and early movement |
The four deformation monitoring methods compared
The four deformation monitoring methods compared here are survey prisms, drone surveys, slope radar and satellite InSAR. Each entry carries the same fields. The prism and drone rows come first and use Birdi as their worked example; the radar and InSAR rows name typical providers. Every fact was checked on October 7, 2026.
1. Survey prisms (worked example: Birdi)
Best for: 3D movement of known points on an embankment or wall, banded against warning and action levels.
Typical customer size / segment: Tailings and geotechnical engineers at operators, and the consultants who monitor for them.
Key strengths:
- 3D movement, horizontal and vertical, at each point.
- Read continuously by an automated total station, or periodically by survey or drone.
- Simple to compare with warning and action levels set by the engineer.
Access for people outside the paid team: In Birdi, guests view and download without using a paid seat.
Data types in one view: Prism readings, ground control, and in Birdi the drone maps they were read from.
Processing: On site for automated total stations; cloud for drone-read prisms in Birdi.
Watch-outs: Prisms measure only where they are installed, and they can be damaged or destroyed by site work.
Pricing: Prisms and installation, plus a total station or survey program. Birdi's prism reference sets need its Ultimate plan, priced by quote (checked October 7, 2026).
How prism readings work in Birdi
In Birdi, a prism reference set is uploaded to the site once and carried into every new map. Warning and action tolerances are set per site, in meters, and every change is saved as a new version. Each prism movement report records the tolerances in force, and runs from the whole network down to each embankment and prism. Prisms are read as checkpoints, kept out of the solution that positions the map, so they measure movement independently.
2. Drone surveys
Best for: Recording the whole surface between readings, to see where a wall or embankment is changing and how fast.
Typical customer size / segment: Mine sites and consultants with a drone program.
Key strengths:
- Covers the whole embankment or pit wall in one flight, without people at the face.
- Difference grids between surveys show where elevation changed.
- Gives a dated visual record for inspection and reporting.
Access for people outside the paid team: Depends on the platform. In Birdi, guests view without a paid seat.
Data types in one view: Orthomosaics, elevation models, point clouds and difference grids.
Processing: Cloud photogrammetry.
Watch-outs: Periodic, so it misses change between flights. Surface accuracy depends on ground control and is in centimeters, not millimeters.
Pricing: Drone hardware and software. In Birdi, difference grids are on Growth and Ultimate (checked October 7, 2026).
3. Slope radar
Best for: Continuous monitoring of pit walls and dam faces where people or infrastructure are at risk, with alarms.
Typical customer size / segment: Open-pit mines and tailings operators. Typical providers include GroundProbe (Orica) and IDS GeoRadar (Hexagon).
Key strengths:
- Real-time, sub-millimeter deformation monitoring, according to Orica.
- Scans a whole face at each pass, not just fixed points.
- Built-in alarming for early warning.
Access for people outside the paid team: Through each vendor's monitoring software; terms by quote.
Data types in one view: Radar deformation maps and time series.
Processing: On site.
Watch-outs: Measures along its line of sight, needs a clear view of the face, and is a significant capital and operating cost.
Pricing: Hardware purchase or rental, plus monitoring software and services, by quote.
4. Satellite InSAR
Best for: Wide-area and early detection of ground movement across a site, a region or a whole tailings facility and its surroundings.
Typical customer size / segment: Operators, regulators and consultants. Data comes from satellites such as Sentinel-1, processed by specialist providers.
Key strengths:
- Covers very large areas in each pass, including places with no ground instruments.
- Millimeter precision on ground movement, according to Copernicus.
- Works day and night and through cloud, because it is radar.
Access for people outside the paid team: Depends on the provider.
Data types in one view: Ground movement maps and time series for measured points.
Processing: Specialist processing of satellite radar data.
Watch-outs: It measures only at each pass, every few days at best, and only along the satellite's line of sight. Vegetation and fast change reduce its usefulness.
Pricing: Free Sentinel-1 data, plus processing and analysis services by quote.
How do monitoring plans combine the methods?
Monitoring plans combine methods so each covers another's gap. Radar watches the highest-risk face continuously. Prisms give 3D movement at known points. InSAR watches the wider area for early or unexpected movement. Drone surveys record the whole surface and read prisms between other readings. The engineer of record decides the mix.
Under the Global Industry Standard on Tailings Management, the engineer of record recommends the monitoring frequency, and out-of-range performance is handled through trigger action response plans. So the combination is an engineering decision, set out in the monitoring program. Our guide to tailings dam monitoring software covers the tools for each layer.
Matching a method to a risk
- Brittle or fast failure near people: radar, with alarms.
- Known points on an embankment: prisms, read by automated total station or at each survey.
- Wide area or slow regional movement: InSAR.
- The visual record and change between readings: drone surveys.
How accurate are drone surveys for deformation?
Drone surveys measure surfaces to a few centimeters with good ground control, and prisms to a few millimeters on low flights. That suits trends and prism readings, not sub-millimeter, real-time work. Birdi publishes 2 to 4 cm vertical accuracy for surfaces with RTK and ground control, and about 3 to 4 mm for prisms.
The difference comes from what is measured. A prism is a precise target, so its position can be read closely. A surface is built from many pixels, so it carries more noise. Each Birdi prism movement report shows the horizontal and vertical movement accuracy for that capture. For keeping survey control consistent, see our guide to prism survey control.
Frequently asked questions
What is the most accurate way to monitor dam deformation?
Slope radar and automated total stations give the highest precision, at sub-millimeter and millimeter levels respectively, and both can monitor continuously. InSAR reaches millimeter precision over wide areas, but only at each satellite pass. Drone surveys are less precise for surfaces but cover the whole structure between readings.
How does slope stability radar work?
Slope stability radar sends radar signals at a wall or dam face and compares the returns between scans to detect tiny movements, often sub-millimeter. It scans continuously, so it can raise an alarm quickly when movement speeds up. It measures along its line of sight and needs a clear view of the face.
Can InSAR monitor tailings dams?
Yes. InSAR uses repeat satellite radar passes to measure ground movement at millimeter precision over wide areas, and Copernicus lists dams among its uses. It measures only at each pass, every few days at best for Sentinel-1, so it complements rather than replaces radar and prisms.
How accurate are drone surveys for deformation?
With good ground control, drone surveys measure surfaces to a few centimeters. Survey prisms read from low monitoring flights can reach a few millimeters of tagging accuracy in Birdi's published figures. Drones suit trends and prism readings between other methods, not real-time or sub-millimeter monitoring.
How often should prisms be read?
It depends on the structure, its risk and the engineer of record's design. High-risk walls may have prisms read continuously by automated total stations. Lower-risk embankments may be read at each survey, often monthly. Under GISTM, the engineer of record recommends the frequency.
About the author
This guide was written by the Birdi team. Birdi (birdi.io) is a geospatial AI workspace that mining, quarry and construction teams use to share drone surveys, prism records and monitoring data in one map. We checked every product fact against each vendor's own website or help center, and read the ESA and Copernicus sources, on October 7, 2026.
Related reading
- Tailings and wall monitoring software: the tools for each monitoring layer.
- Prism movement reports: warning levels, action levels and a defensible record.
- Tailings dam monitoring explained: the basics of the discipline.
- Prism monitoring and dam safety: how prisms support tailings safety.
Sources
Independent
- European Space Agency and Copernicus. "Sentinel-1 mission." SentiWiki. https://sentiwiki.copernicus.eu/web/s1-mission
- Copernicus Land Monitoring Service. "European Ground Motion Service." https://land.copernicus.eu/en/products/european-ground-motion-service
- Global Tailings Review (ICMM, UNEP and PRI). "Global Industry Standard on Tailings Management." August 2020. https://www.icmm.com/website/publications/pdfs/environmental-stewardship/2020/global-industry-standard-on-tailings-management.pdf
Vendor documentation (read October 7, 2026)
- Birdi Help Center. "How to set movement tolerances and run prism movement reports in Birdi." https://help.birdi.io/en/articles/16951320
- Birdi Help Center. "Understanding accuracy in geotechnical monitoring reports." https://help.birdi.io/en/articles/17125217
- Birdi Help Center. "How accurate are my maps?" and "How to create a difference grid in Birdi." https://help.birdi.io/en/articles/13837409
- Birdi. "Pricing." https://www.birdi.io/pricing
- Orica. "GroundProbe slope stability radars." https://www.orica.com/en/digital-solutions/geosolutions/groundprobe/slope-stability-radars/groundprobe-ssr-sarx
- IDS GeoRadar. "ArcSAR Neo." https://idsgeoradar.com/products/interferometric-radar/ibis-arcsar
- Leica Geosystems. "Leica Nova TM60: Monitoring Total Station." https://leica-geosystems.com/products/total-stations/robotic-total-stations/leica-nova-tm60
