SunPetal

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SunPetal for iPhone

SunPetal measures how many hours of direct sun a particular spot on the ground gets, by sweeping your phone's camera across the sky above it.

Questions, problems, or a scan that looks wrong?
Email hubert.fu@gmail.com. If you can, say which spot and roughly when you scanned it.

Getting a good scan

Stand where the plants will go. Not beside the bed — in it. Two metres (six feet) matters when a tree is five metres away.

Mind the height. The sky is measured from the phone, and your plants are lower: the phone sees over things that shade the bed. Tilt the phone down at the bed for a moment, before or during the sweep, and SunPetal measures how high you are holding it. Then, once the sweep is done, tap Sweep again lower and sweep once more with the phone 30–50 cm (12–20 inches) above the bed. The saved spot can then be read at soil, bed, waist or chest height — pick it from the line under the headline. Both steps are optional.

Do it in daylight, with the sun out if you can. SunPetal fixes true north by finding the sun, which is far more accurate than the phone's compass — magnetometers drift five to twenty degrees near a fence, a car, a patio with rebar in it, or a magnetic phone case. Sweep the camera slowly across where the sun is; don't hold it on the sun. It can cross anywhere on the screen and locks on by itself after a pass or two, and a phone camera is better off not staring at the sun. If the sun is behind cloud or behind a tree you will be offered to tap it instead, or the compass, and the result will say which it used.

Sweep slowly, and sweep the whole band. The meter fills as you cover the part of the sky the sun actually crosses. The Finish button unlocks at 95%.

Why does the same bed give a different answer twice?

Some of it is real and some of it is noise. Two sweeps of one spot taken minutes apart typically differ by a few tenths of an hour a day. The “How sure is this?” section on every result gives the range and breaks down what it is made of.

Larger differences usually mean one of:

The scan says sky where there is a wall

It happens with pale walls, windows and blinds — to a camera they are lit by the sky and genuinely are its colour. Open the spot, tap Something wrong? Fix the map, and draw over anything solid that was called sky. The hours are recalculated and your original scan is kept as it was.

Evergreen and deciduous

SunPetal assumes canopy is deciduous unless you tell it otherwise, because that is the common case and because a bare tree in winter lets roughly half the light through where a leafy one lets 3%. During a sweep, use the Evergreen brush to paint over conifers and anything else that keeps its leaves. It changes the monthly figures, not the summer ones.

“What if a tree came out?”

Use Try removing a tree from the home screen. During the sweep, paint over the tree you are considering felling — only the part of it that would show sky once it was gone. If another tree or a building stands behind it, leave those alone: painting them would open up sky that would still be blocked.

Nothing is felled. The result screen gets a switch, and every figure below it moves when you flip it.

Several spots in one walk

Walk several spots keeps the camera session running between spots, so the phone measures the distance you walked from its own motion tracking — which can see a four-metre (13-foot) step that GPS cannot. Keep the camera uncovered on the way. If the phone loses its bearings the walk ends, and the app will say so; the spots you already saved are unaffected.

If your spots end up in a straight line, add one a couple of metres (about seven feet) to the side: from a line alone, the app cannot tell how far off things are along it.

The map between your spots. Open the walk and tap Sun between these spots. SunPetal moves each spot's sky to wherever you tap — near things swing across it, far things stay put — and paints the sun on the ground between your spots. It takes a few seconds the first time. It only answers within a few metres of a spot you swept, and a tree standing where none of your spots could see it will not appear: where it matters, sweep that spot too.

Adding spots later. On the walk's page, tap Add spots to this walk and look around slowly where the walk went; the phone recognises the garden and carries on from the last spot. If it cannot — the light or the leaves have changed too much — stand exactly on one of the walk's spots, say which, and sweep it again; the new spots are measured from there.

Metres or feet

SunPetal shows lengths the way your phone is set up (Settings › General › Language & Region). To choose, use Lengths at the bottom of the home screen.

Is this a weather forecast?

No. Every figure is a clear-sky figure: the sun that spot would get on a day with no cloud. Cloud can only take it away, so read a date's answer as the ceiling for that date.

What SunPetal cannot do

Privacy

SunPetal has no network code. Nothing you scan leaves your phone unless you share it yourself. See the privacy policy.