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Thread: Ordnance Survey releases map data free

  1. #21
      waldronate is offline
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    It's possible that we're looking at different data sets. I downloaded the "OS Terrain 50" "ASCII Grid and GML (Grid)" data from https://www.ordnancesurvey.co.uk/ope.../products.html and (approximately) followed Getting started with OS Terrain 50 elevation data to convert the data to a single HF2 file. The images below are from that data set (you'll need to view at full res to see the effects because they are usually ten pixels or so across and the thumbnails have been downsampled during post creation).

    Ordnance Survey releases map data free-1-facets.jpg
    Is an example of facets in the data, usually caused by interpolation of vectors or overly sparse data samples.


    Ordnance Survey releases map data free-2-noise.jpg
    Shows some smooth areas and facets, along with the sort of noise that I would expect from electronic collection just ofshore in the south. Areas with rapidly-changing surfaces with respect to a radar or lidar will get this very high frequency noise because the detected maximum shifts from pule to pulse in the sensor. Trees, for example, may give maximum return from their upper canopy, mid-canopy, or even ground, depending on the geometry and type of sensor.


    Ordnance Survey releases map data free-3-smooth.jpg
    Shows a smooth set of mountains in between two other sets. It's quite possible that the terrain really does look like this, but I'm a little suspicious about radically different characteristics in a small span.


    Ordnance Survey releases map data free-3-smooth-phase.jpg
    Is the same as the previous terrain set, but shaded according to the facing angle of the terrain without regard to slope. Higher-frequency elements will show up as more broken areas in this map. The mountains in the center are very smooth, which is quite possible if they are nice rounded terrain in the midst of more broken terrain.

    SRTM data is available at 1km and 90m worldwide (30m in the US, to help foreign powers with flying their terrain-following cruise missiles right to high-value targets). GLCF: Shuttle Radar Topography Mission has the basic description of the SRTM data. The original sets had holes and didn't like steep mountains or flat waterways, but they have been reprocessed a number of times in the last 13 years and merged with other high-quality data sets to give much cleaner results.

    Amusingly enough, OS Terrain 50 | Business and government | Ordnance Survey describes the data as "Vector".

    Please note that I'm saying that the data has no value. I'm just not sure what it offers over the SRTM product and derivatives if the data quality isn't validated. The map is no doubt 50 meters in places, but I can't tell by looking at the data which places are 50m and which are lower.

    The 1970s to 1980s comment turns out to be an error on my part. I must have twitched and scrolled down half a page, because the Land-Form PANORAMA(R) entry right below the OS Terrain 50 data has that notation.

  2. #22
      waldronate is offline
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    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.

  3. #23
      Redrobes is offline
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    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.

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      Redrobes is offline
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    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.

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    Professional Artist Facebook Connected Schwarzkreuz's Avatar
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    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

    {The Mapforge}


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