Ah Brilliant, thanks Joe and Redrobes!
best
Ravs
Ah Brilliant, thanks Joe and Redrobes!
best
Ravs
As a silly side rant, SRTM data was collected in such a manner that the entire world from about 60N to 54S should be reproducable at (at least) 1 arc second (about 30 meter) resolution. Global Elevation Data has a discussion of the beastie. It's purely a limited distribution politics on data that the US taxpayers have already paid to gather. I recall discussions from around the time of collection that said that it was at the request of "not the US" that distribution of the higher-resolution data is limited.
I have been searching for why the fen land seems to be below sea level and then chanced upon this:
Places in England below sea level ? A natural history of Britain -
So its true - Peterborough is about 2.75m below sea level. Which means my map is about accurate then.
As part of the searching I came across the Ordnance Survey user guide to the 50m terrain data and it says that the data has been rasterized from vector data and that the vector data is not contour or grid representation but this TIN format so that it has triangles of spot heights that may run along power lines and so on. So its like contour data but not quite the same. So that clears that up. Why the hell its so bad near to the sea is a mystery tho but I guess with the variable tide and sea level readings and erosion and so on its hard to get an accurate absolute level. It also mentions the problem I discovered with the data where there are mismatches between the tiles for the mean sea level such that the whole tile sea is at a fixed height but two tiles don't have the same value. I had to put some special code into my converter to handle that. Its also true that where my land height is accurate based on mean sea level the coastal shape is not accurate compared to the real sea because of this offset cos the sea shader is always fixed height (1.5m above newlyn datum) in mine. So that clears a few things up for me. I think I can leave this alone for now until I need to use it for something.
Yes I see them in your pictures. I have been scanning around britain and I do see these kind of effects in very flat areas like "The Wash" or the low lands of East Anglia and especially as the land enters the sea but on the whole I rarely see these polygonal facets. I dont see any at the hilly or mountainous regions at all. I reckon its well probable that they were created from the contour lines interpolated so that where its flat you have sparse sampling.
I converted mine to a single HF2 file per coord square so I have 55 x 2000x2000 patches. Im still confused by the below sea level area in East Anglia too. Using a link here it says you can use the OS elevation data and the mouse clicks say 0m or 2m. But if I open the asc file then I get this sort of thing (specifically TL59.asc) "-1.4 -1.4 -1.4 -1.3 -1.3 -1.2 -1.1 -1.1 -1.1 -1 -1 -0.9 -0.9 -0.8 -0.8 -0.7" etc so its in the original data for sure. I can only think that there needs to be some kind of offset but I cant think why.
Just for a quick look back 200 Years ago. A Map of Great Britain`s Ordnance Survey from 1809 (Composite of) Devon Sheets 20 to 27 and title. - David Rumsey Historical Map Collection
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