# Bressay — source and model notes

Built 13 October 2026. **6,000 m radius**, 12,300 m square data envelope, true vertical scale.

## Terrain and context

- Origin: Ofcom radio coordinate 60.130113, −1.096280; grid HU50313874. The TV workbook uses the nearby HU50303870 coordinate.
- [Mapzen/Tilezen Terrain Tiles](https://registry.opendata.aws/terrain-tiles/), zoom 12, retrieved 13 October 2026. Source headers include EU-DEM, GMTED2010 and ETOPO1. Native tile URLs and hashes remain in project source.
- Terrain resampled to 25 m; native resolution and accuracy vary. The regional DEM origin is approximately 200.5 m, while Ofcom lists the station at 224 m. No fabricated correction is applied. Not a local ground survey.
- Buildings, roads, water, coastlines and landmarks: OpenStreetMap API snapshot. Sea polygons closed from mapped coastlines and set to nominal zero; not bathymetry or tides. Cartographic texture, not aerial imagery.
- Buildings use tagged heights, levels × 3 m, or an assumed 7 m. Mapped woodland uses assumed 10 m clusters. These are visual context, not measured obstacles. Simplified building extrusions omit courtyards.
- Surface-height data is unavailable and remains unknown. Terrain-only LOS cannot establish an unobstructed radio link. No Fresnel, earth curvature, refraction or RF propagation calculation.

## Two towers

[Original photographs by James Muir, June 2014](https://tx.mb21.co.uk/gallery/gallerypage.php?txid=904&pageid=1934) distinguish the original radio/main TV tower from the original TV/reserve tower with DAB. Images are not redistributed.

- Original radio/main TV: nominal 71.5 m reference envelope, associated provisionally with radio-tagged OSM node 1644918682.
- Original TV/reserve plus DAB: nominal 70 m envelope, associated provisionally with television-tagged OSM node 1644918677.
- Both mapped locations are used. The functional assignment from tags and historic photos is not a current as-built verification.
- Ofcom BBC FM aerial centre 61 m; SIBC 30 m; BBC DAB 62 m. TV workbook mean aerial height **77 m** conflicts with the historic envelope. The current top arrangement therefore remains unresolved, rather than silently forcing the records to agree.
- Arrays, casings, dishes, platforms, steel widths and feeders are simplified. Exact bearings, dish circuits, vendors, rack inventory and tenant leases remain unknown.

## Landmarks and services

The 6 km area includes Lerwick, Bressay Lighthouse, Noss and Clickimin Broch. The broch at OSM relation 19127610 is shown as a simplified stone ring, not an archaeological survey.

[Ofcom radio](https://www.ofcom.org.uk/tv-radio-and-on-demand/coverage-and-transmitters/radio-tech-parameters) and [TV](https://www.ofcom.org.uk/tv-radio-and-on-demand/coverage-and-transmitters/transmitter-frequency) workbooks were downloaded 3 October 2026 and are dated snapshots, not live feeds. [SIBC](https://www.sibc.co.uk/) checked 13 October 2026. Service carriage is not proof of a separate physical tenant.

## Attribution

© OpenStreetMap contributors, ODbL. European terrain produced using Copernicus data and information funded by the EU (EU-DEM); GMTED courtesy of USGS; ETOPO1 from NOAA; tile assembly Mapzen/Tilezen. [Full provider attribution](https://github.com/tilezen/joerd/blob/master/docs/attribution.md). Ofcom OGL v3.0. Three.js MIT.