Drone flights and orthophotos

Fly on the rail plans drone surveys for the app you have open and keeps them with it: planned, flown, processed. It needs editing rights in the app; readers see the plans and can switch them on to look at them.

#Planning a flight

1. Choose the block. Click a block on the map (a licence, a parcel, any area of a layer), Draw the area corner by corner, or Import a file — a GeoJSON or KML with a polygon.
2. Choose the drone from the catalogue and the Height. The Resolution (centimetres per pixel on the ground), the Lines · spacing, the Photos · interval and the Flight time follow from them.
3. Speed, Front overlap, Side overlap and the Line direction are yours to set; the defaults (75 % / 65 %) are what an orthophoto needs. Empty direction follows the block's long side.
4. Follow the terrain gives every waypoint the altitude that keeps the camera at the chosen height over the ground, from the app's own elevation model or the world model.
5. The block is split into the flights the drone can do on one battery, each with its lines, its photos and its minutes. Save plan keeps it with the app.

A block that is too small for the drone at that height says so; a big block that needs many flights suggests a bigger drone, a higher flight or a smaller block.

#Flying it

The plan is exported for the drone's own app:

• Export KML or Export GeoJSON — the whole plan, with every flight line.
• Area of flight, for the drone's app — one plain polygon per flight, numbered, the way Copterus, DJI Pilot and Litchi read it. Height and overlaps are set in that app to the plan's numbers.

On the phone, the same plan opens in Cartema for iPhone flight by flight, with a pre-flight checklist carrying the plan's own numbers and the distance to the take-off point; the photos you take go up to the flight from there (see Cartema for iPhone).

When the survey is done, set the plan's status to Flown.

#Photos

Add photos on a flight uploads the drone's JPEGs (several hundred at a time is fine). Their position and time are read from the EXIF and shown on the map as dots when the flight is switched on.

Flown already, without a plan? Survey from photos I already have takes the photos first: the block is drawn from where they were shot, a plan is made for it, the photos go up — and the survey is ready to process. Photos without a GPS position in their EXIF cannot be used this way.

#Making the orthophoto

What happens to the photos is done in Studio → Orthophotos; in Fly, a flight with photos has Open in Studio. The orthophoto's page has the map on the right and its tools on the left.

Process sends the photos (at least five) to the platform's processing node. In the background — about twelve seconds a photo — they become:

• an orthophoto, a true-to-scale image of the ground at the flight's resolution;
• a surface model, the heights of everything the camera saw (ground, roofs, trees).

Both arrive as raster datasets of the workspace, listed under Products; the page follows the run and shows its log. Add them to the app as layers like any raster (Building apps in Studio). Make it again repeats the processing, for example after adding photos.

#Putting it in the right place: Align

A drone without RTK places its photos a metre or three off, and the orthophoto inherits the shift. On the orthophoto's page, Align fixes that on the map: switch on a reference you trust among the app's layers (or the satellite basemap), lower the orthophoto's opacity if it helps, then click a point on the orthophoto and where that point really is — a manhole, a parcel corner, a surveyed mark. Each pair is listed with how far it moves; remove one with its ×. One pair moves the image; two also turn and scale it; three or more fit it best. Apply moves the orthophoto and the surface model together.

Back to the image as processed undoes every alignment in one click.

#The photos on the map

On the orthophoto's page, Photos on the map shows every photo where it was taken: Outlines draws each photo's footprint and the camera's position, turned with the drone's heading; Previews lays the photos themselves on the ground, so gaps in coverage show at once. Click a footprint to open that photo, with its height above take-off and heading.

#Starting from photos: New orthophoto

Studio → Orthophotos → New orthophoto starts an orthophoto without a flight plan: a name, the app it belongs to, the drone, and the photos — From this computer (choose the files, or a whole folder as copied from the drone's card) or, for Cartema's team, From the server (a folder already put there). The photos must carry their GPS position; the area is taken from it, and the new orthophoto opens with its tools.

#Control points from the field

For centimetre accuracy and correct heights, the orthophoto is pinned to points measured on the ground. On the orthophoto's page, Control points:

1. Import from the field — choose the surveyor's file (or paste it): a line per point with a name, X (east), Y (north) and Z, separated by commas, semicolons, tabs or spaces; a header line is skipped. Choose its coordinate system (UTM, Israeli TM Grid, Camacupa…), tick North first if the file gives Y before X, and As check points for points kept out to test the result. The points appear on the map at once: if they land far away, the system or the order is wrong.
2. Mark each point in the photos — click a point: every photo that should see it opens side by side, each cut around where the point should be (a dashed ring). Click exactly on the mark in each tile; a cross shows it. Zoom sets how close all tiles are, + / − one tile, ⤢ opens the whole photo for fine work. Every mark you make moves the dashed rings in the other photos nearer the target. Do it in at least three photos per point; the list shows 3/3 when a point is ready.
3. Process — the next run uses every control point marked in three photos, when there are at least three such points (the Process tab says how many). Mark more photos or more points to make it stronger.

A point can be turned into a check point (C/K) or removed at any time.

#Testing the result

On the Test tab, choose Find beside a point and click where its mark shows on the finished orthophoto. Cartema gives the error XY (how far that is from the point's field position) and Z (the surface model's height there against the field height). The check points' RMSE is the accuracy of the orthophoto — the figure to give a client; the control points' RMSE shows how well the model fits the points it was pinned to.

#Good to know

• Resolution follows height: with a small drone, 25 m gives about 0.7 cm a pixel, 80 m about 2.9 cm. Fly lower for detail, higher for area.
• Each flight is processed on its own; a survey of several flights makes one orthophoto per flight.
• Processing uses the platform's own server; nothing leaves it. Photos stay with the flight and go when it is deleted.
• The photos you process count towards the workspace's monthly allowance; beyond it, processing is charged per photo.