Aerial Capture for a Phase 1 or Phase 2 Environmental Site Assessment

Environmental consultants ask for aerial capture on assessment work more often than they used to, and the request usually arrives with an expectation attached. Sometimes the expectation is reasonable and the capture earns its cost in an afternoon. Sometimes it is that a flight will substitute for part of the investigation, which it will not. The gap between those two is worth being blunt about before anybody books a crew.

This is written for the consultant scoping the work and for the site owner or lender paying for it, because both of them have a view on what the imagery is supposed to prove.

What a Phase 1 actually is, and where a flight fits

A Phase 1 environmental site assessment is a judgement exercise built on three inputs: a records and historical review, interviews with people who know the property, and a reconnaissance of the site itself. The output is an opinion on whether potential contamination concerns exist and whether intrusive investigation is warranted. There is no sampling in a Phase 1.

A drone capture touches exactly one of those three inputs. It makes the reconnaissance better. It does nothing for the records review and nothing for the interviews, and those are frequently where the real findings come from. A consultant who turns up with a beautiful orthomosaic and no title history, no fire insurance plans and no conversation with the retired yard foreman has not done a Phase 1.

Within reconnaissance, though, the improvement is real. A walkover of a quarter section of industrial yard produces a set of photographs taken from standing height, in whatever order the walk happened, with no consistent scale and no systematic coverage. An aerial capture produces one continuous image of the whole property at a consistent scale, in one lighting condition, on a known date, which can be measured on and which nobody has to remember the sequence of.

The historical imagery question

This comes up on every Phase 1 and it is worth separating cleanly, because it is the one question a new flight definitively cannot answer.

A Phase 1 wants to know what was on the site before. Where were the tanks, where was the burn pit, where did the wash bay drain, when did the building footprint change, what did the yard look like when it was a different business. Those are questions about the past, and the only honest sources are archives: provincial and federal aerial photography holdings, historical satellite imagery, municipal records, old insurance plans and whatever the owner kept.

A capture flown this month establishes the baseline for every future assessment of the property, which is genuinely valuable and is the reason an owner facing periodic assessments should fly once and keep it. It is not a substitute for the archive search, and nobody should let a flight line item on a proposal quietly displace that search.

Where a current capture does help the historical work is in interpretation. Faint surface features that are ambiguous in a coarse historical photograph often read clearly in a current high-resolution capture, because the ground scar, the differential vegetation or the compacted pad is still there. Pairing an old photo with a new capture of the same ground is a better exercise than reading either alone.

What is visible from the air, and what it means

These are the observations a capture reliably supports on an Alberta industrial, agricultural or commercial property, with the honest caveat attached to each:

• Surface staining and discoloured ground, around loading areas, former tank locations, drum storage, equipment parking and wash areas. It is an indicator of where to look. It is not a confirmed release, and some of it is rust, oil drip from decades of parking, or just a different soil.
• Stressed or absent vegetation in a patch that does not match the surrounding cover. Again an indicator. Compaction, salt from winter sanding, buried gravel and an old building pad all produce the same signature.
• Surface disturbance and fill, meaning areas that have been cut, filled, levelled or capped. Visible, measurable in extent, and useful precisely because it points at the places where buried material is plausible.
• Containers and stored material, drums, totes, bulk bags, tanks, scrap piles and waste accumulations, counted and located across a whole yard without walking it.
• Berms, secondary containment and the state of it, including where a berm has been breached, driven through or never closed.
• Surface drainage and where water goes, which on a flat Alberta site is often the single most useful thing a capture adds, because the migration pathway argument in a Phase 1 depends on it and it is very hard to see from the ground.
• Proximity and adjacent land use, which is part of every Phase 1 and is ordinarily described from a desktop map. A capture shows the neighbouring operation, the shared drainage and the upgradient activity as it actually is on the date flown.

The common thread is that a capture is good at extent, location and condition, and silent on cause and on concentration. Every one of those bullets ends in a sampling decision, not in a finding.

Thermal and multispectral, honestly

This is where ESA-related aerial work gets oversold, so it is worth being direct. A radiometric thermal camera measures the apparent surface temperature of whatever it sees. A multispectral sensor measures reflectance in a few bands and supports vegetation indices. Neither one detects hydrocarbons, salts, metals or any other contaminant.

What they can do is narrow where to sample. Thermal sometimes resolves buried structures and disturbed ground because the thermal mass underneath differs, which is a useful screening result when conditions cooperate. Vegetation indices sometimes map a stress pattern more consistently than the eye does, which helps delineate the extent of something already suspected. Both are strongly affected by soil moisture, recent weather, time of day and season, and both produce false patterns constantly.

If either goes in a scope, it should go in as a screening layer that generates sampling locations, with that framing written into the deliverable so that nobody downstream reads a warm patch as a finding.

Where it fits a Phase 2

A Phase 2 is intrusive. It is boreholes, test pits, soil and groundwater sampling, and lab analysis against applicable guidelines. The aerial contribution changes shape completely at this point, and becomes more useful rather than less.

• A base plan to plot on. A current, georeferenced orthomosaic with a stated coordinate system is a far better background for a sampling plan and a results figure than a screenshot of a web map, and it shows the actual surface features the sampling grid was laid out around.
• A dated surface before any excavation. If remediation follows, the volume of material removed will be argued about. A pre-excavation surface and a post-excavation surface give a defensible number, and the pre-excavation one can only be taken before the machines arrive.
• Delineation support. Once sampling has established a concentration at a point, the extent argument is partly a surface argument: where the fill is, where water moves, where the disturbed ground stops. The capture carries that.
• A record of access and conditions on the dates the field work happened, which matters more than people expect when a report is reviewed years later.

If the imagery is going in a report, the frame has to be stated

An ESA report gets read by a regulator, a lender, a purchaser and sometimes a court. Imagery used in one needs the same documentation discipline as any other measurement in it:

• The date and time flown, and the weather and ground conditions on that date.
• The coordinate system, the vertical datum and the geoid model by name, if anything is measured or plotted.
• How the capture was georeferenced, meaning ground control or a correction service, and the accuracy that was achieved rather than the accuracy the equipment is capable of.
• Unedited originals retained, with metadata intact, separate from anything annotated for the figure.
• Who flew it and under what authorisation.

None of that is onerous, and all of it is the difference between a figure that stands up on review and a nice picture somebody can argue with. We have written separately about what makes a dated capture defensible, and the same requirements apply here.

What it cannot do

The limits are not marginal and they are the reason this article exists:

• Nothing below grade. Buried tanks, piping, foundations, buried drums and historical fill are invisible. Photogrammetry returns the top of whatever reflected light. If subsurface structures are the question, that is geophysics or intrusive work.
• No concentrations. No aerial sensor produces a number a guideline can be compared against.
• Nothing under water or under dense canopy, which on a site with a pond, a ditch system or mature bush removes some of the ground that matters most.
• Nothing inside a building, and in a Phase 1 the building interior, the floor drains and the former process areas are usually where the findings are.
• No records, no interviews, no professional opinion. The assessment is the consultant's work and their liability. The capture is an input to it.

Used with that understood, a flight is a cheap and high-value addition to an assessment: better reconnaissance, a defensible dated baseline, a real base plan for any sampling that follows, and a surface to measure remediation against. Used as a shortcut, it produces a report with a striking figure and a weak basis.