Prepared and maintained by Cambridge Development Archaeologyfor development teams working across Cambridge and Cambridgeshire.
Sources, review dates and corrections →What it measures
Magnetometry, the standard technique for open development sites, detects small variations in the magnetic properties of the soil. Ditches, pits and kilns show up because their fills differ from the surrounding subsoil. The survey produces a plan of anomalies across a large area quickly, without disturbing the ground.
What it does not produce is dates, depths, or confirmation that an anomaly is archaeological. It tells you where to look, not what you have found.
Where it performs well
Across much of the Cambridgeshire claylands and gravel terraces, magnetometry resolves cut features clearly, and a survey can transform an undirected evaluation into a targeted one.
- Large open arable fields with no recent disturbance
- Ditched enclosures, field systems and settlement boundaries
- Sites where the aim is to position trenches rather than to prove absence
- Schemes where an authority will accept a survey-led trenching plan instead of a blanket grid
Where results are weak or misleading
Several conditions suppress or swamp the archaeological signal. In those cases a clean survey plot is not evidence that the site is empty, and treating it as such is how schemes end up with unplanned excavation during groundworks.
- Deep alluvium or peat over fen-edge sites, which can mask features entirely
- Made ground, demolition rubble, buried services and fencing, which produce dominant modern noise
- Stone-built or shallow, ephemeral remains with little magnetic contrast
- Waterlogged deposits and timber structures, which magnetometry does not detect
How it changes your cost
The saving is rarely in the survey itself — it is in what the survey lets you argue. A surveyed site can often justify fewer, better-placed trenches, and gives the archaeological advisor confidence to agree a proportionate scope. On a large site that argument is worth considerably more than the survey fee.
The second saving is in design. Knowing where the dense archaeology sits before the layout is fixed sometimes allows open space, attenuation or landscaping to be positioned over it — preservation in situ, which removes excavation cost rather than reducing it.
The order to do things in
Assessment first, so the survey is targeted and the technique is the right one. Survey second, across the developable area. Trial trenching third, positioned against the survey results and a sample of apparently blank ground, because blank ground is where surveys are most often wrong.
If you have a survey report already, send it with your site plan. The anomaly plan tells us a great deal about the likely mitigation scope and cost band.