# Terrain context — Koh Phangan — Than Sadet–Ko Pha-ngan National Park (OSM relation 7175753)

**Experimental.** The shape of the land around each possible-change site, to help decide what to look for on a field visit. Nothing here has been checked on the ground.

- Elevation is the Copernicus GLO-30 surface model: 30 m cells, and under forest it follows the canopy top, not the ground.
- Valley lines and flow paths are **modelled** from that surface. They show where water would run by the shape of the land. They are not surveyed streams.
- "Modelled upstream land" is the **area of land** the terrain model routes through a site. It is not a measure of rainfall, water volume or stream flow.
- Slope is the median across the site, with the steepest cell in brackets.

## Sites

| Site | Area ha | Elevation m | Slope °, median (steepest) | Modelled upstream land, ha | Sits on (modelled) | Distance to a valley line, m | Flow path out, m | Outlet | Modelled drainage group |
|---|---|---|---|---|---|---|---|---|---|
| 1 | 5.39 | 300 | 19.0 (35.1) | 31.3 | valley line | on one | 3254 | B | Outlet group B/D — near Khlong Bang Nam Khem |
| 2 | 1.72 | 386 | 34.0 (37.4) | 11.5 | valley line | on one | 3626 | B | Outlet group B/D — near Khlong Bang Nam Khem |
| 3 | 1.54 | 344 | 10.8 (13.8) | 0.4 | ridge or upper slope | 210 | 5832 | D | Outlet group B/D — near Khlong Bang Nam Khem |
| 4 | 1.49 | 325 | 29.6 (31.6) | 17.2 | valley line | on one | 3422 | B | Outlet group B/D — near Khlong Bang Nam Khem |
| 5 | 1.48 | 263 | 7.5 (10.6) | 6.8 | hillside | 30 | 3089 | C | Outlet group C — near Khlong Than Sadet |
| 6 | 0.79 | 200 | 9.8 (13.4) | 24.9 | valley line | on one | 1007 | F | Outlet group F — near Khlong Ao Hat Yang |
| 7 | 0.72 | 273 | 7.9 (13.1) | 0.2 | ridge or upper slope | 192 | 3252 | C | Outlet group C — near Khlong Than Sadet |
| 8 | 0.93 | 302 | 14.6 (18.6) | 3.2 | hillside | 102 | 3956 | A | Outlet group A |
| 9 | 0.7 | 245 | 12.8 (21.3) | 0.3 | ridge or upper slope | 307 | 3939 | C | Outlet group C — near Khlong Than Sadet |
| 10 | 7.41 | 169 | 14.1 (26.0) | 246.4 | valley line | on one | 2648 | A | Outlet group A |
| 11 | 2.61 | 194 | 13.1 (20.1) | 4.0 | hillside | 252 | 3504 | C | Outlet group C — near Khlong Than Sadet |
| 12 | 1.38 | 404 | 9.2 (12.2) | 3.6 | hillside | 325 | 1719 | E | Outlet group E |
| 13 | 1.34 | 277 | 21.9 (30.1) | 0.5 | ridge or upper slope | 345 | 3079 | B | Outlet group B/D — near Khlong Bang Nam Khem |
| 14 | 1.05 | 259 | 23.4 (33.8) | 2.8 | hillside | 157 | 4586 | D | Outlet group B/D — near Khlong Bang Nam Khem |
| 15 | 0.72 | 216 | 15.9 (19.0) | 22.1 | valley line | on one | 2565 | G | Outlet group G — near Khlong Ta Pan |
| 16 | 0.68 | 402 | 8.6 (15.7) | 0.5 | ridge or upper slope | 360 | 2223 | H | Outlet group H |
| 17 | 0.65 | 246 | 10.2 (23.4) | 0.7 | ridge or upper slope | 187 | 3819 | C | Outlet group C — near Khlong Than Sadet |
| 18 | 0.65 | 207 | 25.3 (33.0) | 61.2 | valley line | on one | 3114 | B | Outlet group B/D — near Khlong Bang Nam Khem |

## Modelled drainage groups

Sites grouped by the outlet their modelled flow path reaches. Outlets that sit by the same named mapped waterway are shown as one group, because on flat ground the model can route one real river to two neighbouring outlets. Whether they are the same river in reality has not been verified.

| Group | Modelled outlets | Sites | Their combined area, ha |
|---|---|---|---|
| Outlet group B/D — near Khlong Bang Nam Khem | B, D | 1, 2, 3, 4, 13, 14, 18 | 13.18 |
| Outlet group A | A | 8, 10 | 8.34 |
| Outlet group C — near Khlong Than Sadet | C | 5, 7, 9, 11, 17 | 6.16 |
| Outlet group E | E | 12 | 1.38 |
| Outlet group F — near Khlong Ao Hat Yang | F | 6 | 0.79 |
| Outlet group G — near Khlong Ta Pan | G | 15 | 0.72 |
| Outlet group H | H | 16 | 0.68 |

## Where the water leaves

| Outlet | Location | Leaves into | Modelled upstream land, ha | Nearest mapped waterway, m | Its name | Sites |
|---|---|---|---|---|---|---|
| A | 9.77781, 100.05568 | sea | 470.2 | 38 | — | 8, 10 |
| B | 9.69485, 100.03519 | sea | 602.3 | 26 | Khlong Bang Nam Khem | 1, 2, 4, 13, 18 |
| C | 9.74953, 100.07474 | sea | 1409.2 | 26 | Khlong Than Sadet | 5, 7, 9, 11, 17 |
| D | 9.69567, 100.03246 | sea | 864.4 | 279 | Khlong Bang Nam Khem | 3, 14 |
| E | 9.68777, 100.04419 | sea | 26.6 | 686 | — | 12 |
| F | 9.71968, 100.07628 | sea | 233.6 | 48 | Khlong Ao Hat Yang | 6 |
| G | 9.77238, 100.05621 | sea | 312.5 | 10 | Khlong Ta Pan | 15 |
| H | 9.68198, 100.07425 | sea | 93.9 | 8 | — | 16 |

## How far to trust it

Compared with 77.0 km of rivers and streams mapped in OpenStreetMap, which the model never used: 63% of mapped waterway length lies within 60 m of a modelled valley line. For comparison, 24% of all land does. The typical gap is 42 m.

This is consistent with the model capturing broad valley structure, but it does not establish the accuracy of individual drainage lines: mapped rivers lie in valleys and so do modelled lines, and OpenStreetMap is itself incomplete, mostly in the uplands where the sites are. A site marked "valley line" is a place to look for a channel, not a place where one is known to exist.

A valley line is drawn where the model routes at least 10 ha of land through a cell. "Ridge or upper slope" means under 1 ha. Outlets within 200 m of each other are one outlet; an outlet takes the name of a mapped waterway within 300 m.