Buying aerial imagery, lidar, and change detection services

Reference article · 6 min read

Aerial imagery and lidar are among the few data purchases a local government makes that nearly every department eventually touches. Assessors use imagery for valuation review, public works uses it for asset inventory, planning uses it for permit and code enforcement, stormwater uses elevation data for drainage modeling, and emergency management uses both for pre-incident planning and damage assessment. A specification written by one department to solve one problem often yields data the rest of the organization cannot use, and the gap is not discovered until the next budget cycle.

These acquisitions are also episodic. An agency may fly once every two or three years, so the staff writing the solicitation frequently did not write the last one. What follows is a practical framing for scoping an acquisition, verifying what arrives, and setting license terms that hold up for the full life of the data.

Start from the decisions the data has to support

Before writing a single specification, list the uses and the tolerance each one needs. Parcel and valuation review, engineering design, floodplain and stormwater modeling, canopy assessment, and impervious surface billing all have different requirements, and the most demanding one usually sets the floor. If the most demanding use is narrow, consider a tiered acquisition: tighter specifications over the developed core and looser ones over rural or undeveloped areas. Say so explicitly in the solicitation rather than leaving vendors to guess where the money should go.

Write resolution and accuracy as numbers, not adjectives

"High resolution" is not a specification. Name the measurable parameters and the standard you are testing against.

• Ground sample distance. State the nominal pixel size for orthoimagery in ground units. Finer GSD raises flight time, storage, and cost, so match it to the smallest feature you need to identify.
• Horizontal accuracy. Reference the ASPRS Positional Accuracy Standards for Digital Geospatial Data and state the required RMSE in ground units. Legacy map-scale language alone leaves too much open.
• Lidar density and vertical accuracy. Specify aggregate nominal pulse density and vertical accuracy using non-vegetated and vegetated accuracy classes. The USGS 3D Elevation Program Lidar Base Specification and its quality levels are a common reference point for public agencies; it is revised periodically, so cite the current version.
• Spatial reference. Name the horizontal and vertical datum, state plane zone, geoid model, and units. The National Geodetic Survey has been modernizing the national reference system, so confirm current guidance and plan for how a future transition would affect your holdings.
• Bands and bit depth. If you need vegetation indices or automated classification, require a near-infrared band and adequate radiometric depth up front.

Match timing to the product, not to the calendar

Leaf-off flights, generally between leaf drop and bud break, give clean ground visibility, better lidar ground returns, sharper building edges, and better drainage interpretation. Leaf-on flights support canopy mapping, vegetation health, and tree inventory work. If you need both, that is two flights or a deliberate compromise, and the budget should reflect the choice.

Write the window as a date range plus condition clauses: snow-free ground, specified sun angle minimums, cloud and shadow limits, and acceptable water levels for hydrographic breaklines. Include what happens when weather closes the window, including whether the schedule slips to the next season and how that affects payment and delivery dates tied to your assessment or budget calendar.

Know what you are buying beyond the orthophoto

• Orthophoto versus true ortho. Standard orthophotos still show building lean. True orthos remove it at higher cost and with denser control and overlap, which matters most in dense urban cores.
• Oblique imagery. Angled views support assessor field review, code enforcement, and fire preplanning, and measurement usually happens inside vendor software. Ask what you retain if you do not renew.
• Lidar derivatives. Classified point cloud, bare-earth terrain model, surface model, contours, and breaklines are separate deliverables with separate effort. Name the classification scheme you expect.
• Planimetrics and analytics. Building footprints, impervious surface, canopy, and change detection are labor-intensive. Specify minimum mapping unit, classification rules, and whether change detection is a delivered geospatial layer or a summary report.

Licensing, delivery, and hosting

Ask for a perpetual, non-exclusive license covering every department, elected office, and contractor acting on the agency's behalf, plus the right to share with other public agencies, publish through a public web map, and release as open data if you choose. Confirm the agency owns derivative works. For subscription imagery, get clear in writing what you keep when the subscription ends, and understand that data you hold may be subject to public records obligations regardless of license language. Records law varies by state, so have counsel review the terms.

Specify open, durable delivery formats such as cloud-optimized GeoTIFF for imagery and the ASPRS LAS or LAZ format for point clouds, with metadata records in a standard schema. If the vendor hosts services, require documented OGC endpoints, stated availability, authentication details, and a copy of the source data delivered to the agency independent of the hosted service.

Consortium buys and recurring cycles

Many states run coordinated imagery or elevation programs, and federal elevation partnerships have offered cost-share opportunities; availability and terms change year to year, so confirm with your state GIS coordinating body and the relevant federal program office. Even without a formal program, neighboring jurisdictions flying together can share mobilization and produce consistent data across shared boundaries.

If change detection is a goal, comparability matters more than any single year's quality. Keep specifications, spatial reference, and processing consistent across cycles, and consider a multi-year contract with priced option years rather than repeated one-off procurements.

What to require before you accept delivery

1. Independent checkpoints surveyed by a licensed land surveyor, distinct from any points used for sensor calibration, distributed across the project area and land cover types per the referenced standard's minimum counts.
2. A written accuracy report, flight logs, calibration records, and complete metadata as a condition of final payment.
3. Visual quality review for tonal balance, seam line placement, smearing, cloud and shadow, missing tiles, lidar voids, misclassified points, and water surface treatment.
4. Retainage held until acceptance testing is complete, with a defined correction period and reflight remedy for areas that fail.
5. A named agency technical reviewer and a realistic internal review window, since acceptance testing is agency work that has to be staffed.

HQT is a technology and software company serving United States state and local government agencies, with work spanning data and analytics and systems engineering of the kind these acquisitions depend on.

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