Generate contours: boundary, interval and smoothing

Tap Draw Boundary

It's the right-hand button in the Define field boundary? window. The window closes and the map is ready for you to tap corners. Skip (use full extent) on the left goes straight to Contour Settings with no boundary.

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Tap each corner of the field

Tap the map at one corner of the field, then go round the edge tapping each corner in turn. An orange line joins them up, and the top button in the right-hand column counts the corners: Done (4 pts). Tapped in the wrong place? Undo, just under it, takes the last corner off.

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Keep it simple: an outline that crosses itself is refused (Polygon edges cross. Adjust your boundary.). Undo back to the bad corner and go round the other way. To zoom or move the map, pinch or drag with two fingers (pinch to zoom).

Tap Done

Tap Done (4 pts) at the top of the right-hand column (it needs at least three corners). The outline turns cyan and Contour Settings opens. Cancel, under Undo, throws the corners away and stops drawing.

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Check the top line

Under the Contour Settings title: the size of the surface and its elevation range. The range is worked out as if the elevations were metres: right for a Saved LiDAR DEM, about three times high for a GeoTIFF or point file in feet. Tapping Meters or Feet doesn't change it.

Tap an interval

The interval is the height from one contour to the next. Tap one of the chips under Contour Interval; the one you pick turns green. A smaller interval gives more lines, closer together: pick it to suit how tightly you need to hold elevation. 3″ is picked to start.

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In metric the chips read 25, 50, 75, 150 and 300 mm; with decimal feet 0.08, 0.17, 0.25, 0.5 and 1 ft (the same heights).

Or type your own in Custom

For a size that isn't on a chip, tap the Custom box under the chips and type the number (typing); here 36 inches. A GeoTIFF or point file also shows Source units under it: tap what the file really is, Meters or Feet (why it matters).

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Set the smoothing

Drag the dot on the Smoothing slider: right for more, left for less. It has nine steps from None to Max, with the name shown on the right; Medium to start. More smoothing rounds off bumps from rough or sparse data but loses small detail; less keeps detail but can give jagged lines.

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Check the estimate, then tap Generate

The grey box shows about how many contours you'll get. Over 200 it turns amber with Many contours — consider a larger interval. Over 500 it turns red and Generate greys out until you pick a larger interval. When it's fine, tap Generate at the bottom right: Generating contours... runs, longer for big fields.

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The estimate is a guide. The count on the map can come out higher, because a contour broken into pieces counts each piece.