Glossary
AGL (Above Ground Level)
Height measured from the ground directly beneath the aircraft rather than from sea level — the reference Transport Canada uses for the 400 ft ceiling, and the setting that decides swath, pattern, and map resolution on every agricultural flight.
Definition
AGL is altitude referenced to the terrain immediately below the drone, as distinct from ASL (above sea level), which is referenced to a fixed datum. On rolling ground the two diverge constantly: fly a straight line at a fixed ASL altitude across a hill and your AGL height drops as the land rises beneath you. Agricultural drones resolve this with downward-facing radar or terrain-follow from a loaded elevation model, holding a commanded AGL height as the ground moves under the aircraft. Which reference a number is quoted in matters, because almost every operational limit and every application setting is written in AGL.
In an Alberta Context
Two AGL numbers matter on an Alberta farm and they are far apart. The regulatory one is the ceiling: Transport Canada’s Part IX rules cap routine remotely piloted operations at 400 feet AGL, and going above it requires specific authorization. The operational one is the working height, which for spreading and spraying is low — metres above the canopy, not hundreds of feet — because that is where the deposition pattern behaves. Mapping flights sit far higher than application passes but still well under the ceiling. Alberta’s rolling parkland and coulee country is exactly the terrain where holding a commanded AGL matters: a pass flown at constant altitude instead of constant height will spread wide over the low ground and tight over the knolls.
Why It Matters
AGL is the single setting that ties the legal envelope to the agronomy. Fly higher on a spreading pass and the granules disperse over a wider footprint, which sounds like fewer passes but in practice broadens and flattens the deposition curve — which is why effective swath has to be set from a pan test at the height you will actually fly, not from a brochure figure or a different altitude. Fly lower and the pattern tightens but you cover less per pass and have less clearance over obstacles. On mapping work the relationship is more direct still: ground sample distance scales with height, so doubling AGL roughly doubles the ground each pixel covers and halves the detail in the map. Change the height and you have changed the job — which is why a repeatable AGL, held over uneven ground, is what makes a result repeatable.
Frequently Asked Questions
+What does AGL mean on a drone flight?
AGL stands for above ground level — height measured from the terrain directly beneath the aircraft, rather than from sea level. It is the reference used for regulatory height limits and for the working height of a spreading, spraying, or mapping pass. On rolling ground an aircraft holding a fixed AGL height climbs and descends with the land.
+What is the maximum AGL height for a drone in Canada?
Transport Canada’s Part IX rules cap routine remotely piloted operations at 400 feet AGL. Operating above that requires specific authorization. Agricultural application work is flown far below the ceiling anyway — spreading and spraying passes run low over the canopy, because that is where the deposition pattern is controllable.
+How does flight height affect a granular spread pattern?
Height changes the shape of the deposition curve. Flying higher lets granules disperse across a wider footprint, but the pattern also broadens and flattens rather than simply getting wider at the same intensity. That is why effective swath is set from a pan test conducted at the height, product, and rate you will actually fly — change the AGL and the tested swath no longer applies.
+What is the difference between AGL and ASL?
AGL is referenced to the ground immediately below the aircraft; ASL is referenced to sea level. Over flat ground they track each other, but over a hill an aircraft holding a constant ASL altitude loses AGL height as the terrain rises. Agricultural drones hold AGL using downward radar or a loaded elevation model so the working height stays constant across uneven fields.
+Does AGL affect drone map resolution?
Yes, directly. Ground sample distance — the ground area each pixel covers — scales with height, so flying twice as high roughly doubles the GSD and halves the detail. Lower flights produce finer maps but cover less ground per pass, so mapping height is chosen as a trade between the resolution the job needs and the acres to be covered.
+Why does a repeatable AGL matter across seasons?
Because comparability depends on it. Two multispectral flights over the same field are only fairly compared if they were flown at the same height, and a spread pattern is only reproducible if the height that was pan-tested is the height flown. Recording and holding a commanded AGL is what turns a single good result into a repeatable one.