Sun hours map
The sun hours map computes how many hours of direct sun each point on the ground receives on the chosen date and gives it as a plate and three values.
Overview
The «3D model» on the «Sun & shadow» tab shows the shadow at one time of day. It does not say how many hours of sun a point receives over a day.
The sun hours map solves the shadow every 10 minutes from sunrise to sunset and adds up, at each point on the ground, the time in sun. The same calculation appears as the SUN HOURS plate in the diagram set (winter solstice).
How to use
- Set the site and the radius
- On the «Sun & shadow» tab, choose the «Sun hours map»
- Choose the date (21 June, 21 March, 23 September or 21 December)
- Press «Compute sunlight hours»
- Save with «Save PNG» or «Save plate only»
«Save PNG» keeps the title, colour bar and note. «Save plate only» saves the drawing alone; add the caption yourself.
Values on the sheet
| Value | Definition |
|---|---|
| Daylight | Time from sunrise to sunset on the chosen date |
| Mean sun hours | Mean daily sun hours on the ground inside the plate |
| Under 1 h | Share of the ground that receives less than 1 hour of sun in the day |
The values compare positions. They are not a planning criterion. The Korean edition's sunlight criteria are not shown in the UK edition.
In the UK, sunlight to gardens and open spaces is usually assessed against the BRE guide Site layout planning for daylight and sunlight (BR 209), which recommends that at least half of an amenity area receives at least two hours of sunlight on 21 March. The plate does not apply this test and does not replace it (see Limitations).
Time reference
The map sums the whole day from sunrise to sunset. It does not cut a time window.
Sun positions are computed in true solar time (12:00 is when the sun is due south). London lies close to the Greenwich meridian, so solar noon falls within about a quarter of an hour of 12:00 GMT through the year. Times are shown in GMT (UTC+0) without British Summer Time: in summer, 12:00 GMT is 13:00 on the clock.
Solar noon is computed from the site longitude and the time zone in the header of the EPW weather file (0 for London), not estimated from the longitude.
Method
The map uses the same building height map as the wind and thermal comfort sheets, plus the terrain. Shadow is pure geometry: no CFD and no GPU job.
- For the chosen date, the sun position is computed every 10 minutes from sunrise to sunset
- At each step, every ground cell is classed as in sun or in shade
- The time in sun is summed for each cell
The difference between 10-minute and 6-minute steps is 0.01 h.
Shadow test
The grid is walked away from the sun, carrying the highest point seen so far minus the drop over distance (a running maximum). No building between samples is skipped, and the test is 30% faster than sampling along rays. The calculation is limited to the plate; leaving out the buffer ring outside it reduces the work by a factor of 5.5 without reducing precision. The thermal comfort sheet uses the same shadow function.
Compute time
| Radius | Compute time |
|---|---|
| 300 m | about 4 s |
| 500 m | about 14 s |
| 800 m | about 28 s |
Why a horizon map is not used
A horizon map (skyline height by direction, baked once per cell) makes any date and time free after 4–9 s of baking. Over a day it returned 0.15–0.29 h more sun than the correct value: each instant was off by 1–5%, always in the same direction, because the distance sampling along each direction stepped over narrow buildings. A bias towards more sun is not acceptable, so the method is not used.
Display
Colour breaks are at 0, 1, 2, 3, 4, 5, 6, 8, 10 and 12 h. Up to 6 h the bands are 1 hour wide, so points with little sun are separated by colour. Buildings are filled pale grey.
Limitations
This plate is not a daylight and sunlight assessment. Such assessments use survey data and validated software.
- Heights come from Environment Agency LiDAR flown 2016–2022. A building completed after the survey carries a fallback height (OpenStreetMap height, storeys × 3 m, or 3 m) and can cast a shorter shadow than it does, so those spots can read sunnier than reality
- The plate measures sun on the ground. Windows are vertical, receive the sun at another angle and differ by floor; the value on the ground says nothing about the rooms above it
- Trees are not included. Tree shade is used only on the thermal comfort sheet
Use the plate at concept stage to see where winter sun reaches around the site and where it does not: where outdoor space will be usable in winter, which ground-floor frontages stay dark, and which parts of the site are shaded by neighbours all day.
Data sources
| Data | Use |
|---|---|
| Overture Maps (© OpenStreetMap contributors, ODbL) footprints · Environment Agency LiDAR Composite 1 m (DSM − DTM, OGL v3) | Building height map, shared with wind and thermal comfort |
| Environment Agency LiDAR DTM (Copernicus GLO-30 in gaps) | Terrain |
| EPW header (Climate.OneBuilding.org TMYx, London Weather Centre, St James's Park, 2011–2025) | Time zone for solar noon |