What the terms in your quote actually mean
Aerial quotes are full of technical language, and you should be able to read one without taking anything on trust. These are the terms that show up most, defined plainly.
- Ground sample distance (GSD)
- The real world size of one pixel in an aerial image, usually written in centimetres per pixel. A 2.5 cm/px capture means each pixel covers 2.5 cm of ground. Lower is more detailed and requires flying lower or longer. GSD describes detail, not positional accuracy, and the two are routinely confused.
- RMSE
- Root mean square error. A single number summarising how far a mapped position sits from its true position, calculated across independent checkpoints. A reported 0.01 m RMSE means the capture was checked against known points and the residual error was measured rather than assumed.
- RTK
- Real time kinematic positioning. The aircraft receives correction data during flight and records each image position to centimetre precision, which removes most of the need for ground control targets.
- NTRIP
- The protocol that streams RTK corrections over a mobile network from a base station network rather than a base station you set up yourself. In Alberta this makes real world coordinate accuracy achievable without hauling out a base station and establishing its position yourself.
- Ground control point (GCP)
- A marked target on the ground whose position is measured independently, used to georeference or to check a capture. With RTK the role shifts from georeferencing to verification.
- Orthomosaic
- A single large image built from hundreds of overlapping aerial frames, corrected so that scale is consistent across the whole picture. Distances measured on an orthomosaic are true distances, which is what separates it from an aerial photograph.
- Photogrammetry
- Deriving three dimensional measurements from overlapping two dimensional photographs. The basis of most drone mapping, and considerably cheaper than lidar on open ground.
- Lidar
- A sensor that measures distance with laser pulses. Its advantage over photogrammetry is the ability to return ground points through vegetation and tree canopy, which makes it the sensor of choice for wooded terrain, corridors and reclamation work. The two are frequently flown together on a single mobilisation.
- Point cloud
- Millions of individually positioned three dimensional points describing the surface of a site, delivered as LAS or LAZ. The raw material behind terrain models, contours and volumes.
- DSM and DTM
- A digital surface model includes everything the sensor saw, buildings and vegetation included. A digital terrain model is the bare ground with those objects removed. Volumes and grading work need the terrain model, not the surface model.
- Overlap
- How much each aerial frame shares with the next, given as front and side percentages. A capture at 90 percent front and 85 percent side overlap produces far more redundancy and a cleaner result than a sparse flight, at the cost of flight time.
- AGL
- Above ground level. Flight altitude measured from the ground beneath the aircraft rather than from sea level. A thermal capture at 100 m AGL yielded a 13 cm/px thermal ground sample distance.
- Radiometric thermal
- Thermal imagery where every pixel stores an actual temperature value rather than only a colour. It allows findings to be reported as measurements and compared between flights.
- NDVI and NDRE
- Vegetation indices calculated from multispectral imagery. NDVI is strongest in early growth and saturates once the canopy closes. NDRE uses the red edge band and stays useful later in the season on a dense canopy.
- Multispectral
- A sensor that records specific narrow bands of light, including bands outside human vision, which is what makes crop stress visible before it is visible to the eye.
- RPAS
- Remotely piloted aircraft system. The term Transport Canada uses in Canadian regulation for what most people call a drone.
- Advanced operations certificate
- The Transport Canada certification required to fly in controlled airspace, near people, and near aerodromes. Basic operations certification does not permit that work.
- Orthorectification
- Correcting aerial imagery for terrain relief and camera tilt so that the resulting image has uniform scale and can be measured on.
Reading a quote and something in it does not add up? Send it over. We will tell you what the terms mean for your project, whether or not the work comes to us.
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