What it does
Reef and benthic surveys often estimate cover by photographing quadrats along a transect and identifying what lies under a set of points on each photo, the method made popular by the program CPCe. This page does the same in your browser:
- Points. Each photo gets points laid out as stratified random (the photo is split into rows × columns and one point falls at random in each cell), simple random (n points anywhere) or a regular grid (cell centres). A border can be kept clear of the frame.
- Labels. You pick a point and tap its category. Categories are your own list, one per line; the first nine also have number keys.
- Cover. Cover of a category = its points ÷ all counted points × 100, pooled over every photo, with a Wilson binomial confidence interval. Categories such as shadow, tape or “can’t tell” can be left out of the total. A second table gives each photo’s own cover.
The photos stay on your device; nothing is uploaded.
How to use it
- Photos. Set the point layout, then press Add photos and choose one or several quadrat photos.
- Points. Check the points on the first photo. Place points again gives that photo a fresh layout from the current settings.
- Label. The highlighted point is circled. Tap its category (or press its number); the page jumps to the next unlabelled point. Tap any point on the photo to go back to it. Switch photos with the Photo list.
- Cover. Read the tables as you go, and download the cover table, every point, or a report.
What each option does
- Points per photo / rows and columns. More points narrow the interval. 25 to 50 per photo are common; the number of photos matters as much as the points per photo.
- Border left clear. Keeps points off the quadrat frame and the photo’s edge.
- Random seed. Photo 1 uses the seed, photo 2 the seed + 1 and so on, so you or a colleague can recreate exactly the same points.
- Left out of cover. Points with these labels are not counted in the total, so cover is of the visible seabed.
- Interval. 90, 95 or 99 % Wilson interval. It treats every point as independent, which is optimistic when points on one photo are alike; for a transect, the spread of the per-photo covers is the more honest error.
Worked example
The example has two made-up quadrats with 5 × 5 stratified random points each (seeds 1 and 2), already labelled from the drawing. One point falls on the tape and is left out, leaving 49 counted points. The page shows Sand: 53.1 % (95 % CI 39.4 to 66.3 %) as the largest category, and hard coral at 26.5 % (13 of 49 points, 95 % CI 16.2 to 40.3 %), algae 14.3 % and rubble 6.1 %. Soft coral was missed by every point: 0 %, but with an upper limit of 7.3 %, a reminder that 50 points cannot rule out a rare category.
Tips
- Photograph every quadrat from the same height, straight down, and include the frame so you can crop to it.
- Decide the category list and the rules for hard cases (bleached coral, turf on dead coral) before you start, and keep them for the whole survey.
- Stratified random points spread out more evenly than simple random ones and usually give a slightly more precise cover for the same work.
- Report the number of photos, points per photo and the layout with your cover values.
FAQ
Can I use it for seagrass, kelp, rocky shores or plant quadrats? Yes. Type any categories; the method is the same for any photo quadrat.
Are my labels saved? Only on this page while it is open. Download the point CSV to keep your work.
Why does cover come with an interval? Points are a sample of the photo. With 50 points, a true cover of 25 % will often read anywhere from about 15 to 40 %.
Where the numbers come from
- Random point counts on photo quadrats as in Kohler, K. E. and Gill, S. M. (2006) Coral Point Count with Excel extensions (CPCe). Computers & Geosciences 32, 1259–1269. fibub's page is independent and uses none of CPCe's code or files.
- Wilson, E. B. (1927) Probable inference, the law of succession, and statistical inference. Journal of the American Statistical Association 22, 209–212 (the interval), checked in the tests against statsmodels 0.15 proportion_confint(method='wilson') to 1e-6.
- Point layouts (simple random, stratified random with one point per cell, regular grid) use a seeded random number generator (mulberry32), so the same seed gives the same points.
- The two example quadrats are drawn by the page from made-up shapes, and their points are labelled from the drawing.
- Photos you open are read on the page only. They are never uploaded or stored.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-09.