Ground you have not flown
You can run analyses over public aerial imagery without flying anything.
Open Base imagery, draw the ground you are interested in, and the platform pulls that imagery from a public provider and makes a survey out of it. From there it behaves like any other survey: analyse it, run a trained model over it, export the result.
It is useful for looking at ground before you commit an aircraft to it, for country you cannot get to, and for a quick answer on somebody else's paddock in a meeting.
Drawing the area
Rectangle or polygon — your choice, and the toolbar has both. A polygon is worth the extra clicks when the ground you care about is a shape: it fetches less imagery, takes less time, and the result is not padded with country you did not ask about.
The two things this makes weaker
There is no elevation. A flight produces a three-dimensional reconstruction because the same ground appears in many frames from different positions. A basemap is one flat picture. So every analysis that reads the shape of the ground — stockpile volume, slope, ponding, watercourses, gullies, and the trench half of the pipe finder — cannot run on it at all. What remains is what can be read from colour: trained models, and the vegetation signal.
It is much coarser. A flight at 100 m gives you two or three centimetres per pixel. Public imagery over rural Australia is typically 30 cm to a metre or worse. That is a factor of twenty, and it decides what can be seen: a mob of cattle, yes; an individual weed, no.
A model trained on flight imagery will find far less on a basemap, because the detail it learned from is not present. That is not a fault in the model.
When the detail is not there
Providers serve tiles beyond the ground they actually hold imagery for, by enlarging coarser tiles or by returning a placeholder that says nothing useful. Both come back as a normal, successful response and a smooth, plausible, entirely invented picture.
The platform checks before it fetches. If the detail you asked for was never captured over that ground, it drops to the finest that genuinely exists and tells you what it used. If nothing usable exists at all, it says the imagery is not available at that resolution rather than handing you an invention.
Believe it when it says so. Inland Australia is often only held at a moderate zoom, and a request for the finest available will routinely come back coarser.
Dates
The imagery was captured at some point. The provider does not reliably say when, and it can be several years old. It is not a picture of today, and you should not use it to establish that something was or was not there on a particular date.
The size limit
One extraction may cover up to 1 km² — about a kilometre square — on every plan except Enterprise.
This limit is different in kind from the others. It is not about what you have bought: the imagery belongs to a tile provider, is served to us on their goodwill, and dragging a rectangle over a shire costs them thousands of requests while feeling like one click. Enterprise plans are uncapped, and individual operators can be exempted by arrangement.
What you may do with it
Analyse it here. That is what the providers' terms allow, and it is what the feature is for.
You may not redistribute or resell the imagery, or anything derived from it that contains the imagery itself. Your findings — the outlines, the counts, the measurements — are yours. The picture underneath them is not.
Attribution is shown wherever the imagery appears and must not be removed.
Sources
- Esri World Imagery — worldwide, and the default.
- USGS aerial — United States only, public domain, and generally finer than the alternative where it exists.