FAQs
How Did You Do It?
Tom and Ryan go into detail on their respective blogs:
- tombh.co.uk/packing-world-lines-of-sight
- tombh.co.uk/longest-line-of-sight
- ryan.berge.rs/posts/lines-of-sight
- ryan.berge.rs/posts/total-viewshed-algorithm
What Assumptions Did You Make?
- The original source of our data is from NASA’s SRTM survey which is ~100m resolution analysis of the planet’s elevation data. It is known to have some issues so we used a cleaned version kindly provided by viewfinderpanoramas.org.
- We used a globe earth meaning it is an approximation of earth’s shape, as it is an oblate spheroid.
- We take refraction into account, and we use what the GIS community has calculated to be the world average, which is a refraction coefficient of
0.13. - The height of the observer is
1.65mor5'5". - Each viewshed is calculated using 360 lines of sight each seperated by 1°. This could potentially miss some longest lines of sight, but it is considered to be the optimal resolution to balance the accumulation of errors and computational costs. For more details, see: Siham Tabik, Antonio R. Cervilla, Emilio Zapata, Luis F. Romero in their 2014 paper Efficient Data Structure and Highly Scalable Algorithm for Total-Viewshed Computation https://ieeexplore.ieee.org/document/6837455
- All computation is done on AEQD reprojections of the raw data. For the longest lines of sight on the planet, ~500km, the worst case errors caused by this projection can reach ~0.0685%. This error is only relevant to viewsheds at the edge of the computable area of tiles, therefore those viewsheds around 500km from the centre of tiles.
Is The Source Code Available?
Yes. The core algorithm. The pipeline and web app.
Why Don’t You Make The Sea More Visible?
The sea actually has above average visibility, due to it generally not being blocked by hills and mountains. As such it can generate some often interesting and sometimes beautiful heatmaps, like around the islands of South Korea.