SRTM data

[Guest Jozef_K]

Which area is covered by SRTM mesh data files?When I d/l let's say S18W068.HGT.ZIP (which contains the file S18W068.HGT) which area is then covered by these data. I know, S18-W068 is one of the corners of a one degree long./lat. rectangle. But streches it from S18 to S19 or to S17. And W068 to W069 or W067?Does it make any difference for north/south and east/west hemisphere?I mean, when S18W068 is e.g. the lower left corner, is it then always the lower left corner no matter which hemisphere?Can anyone could enlighten me? Thanks in advance.Jozef http://www.dse.nl/~joker32/pictures/signature.jpg

[Holger 0]

Hi Jozef:from the SRTM docs:"3.0 Data FormatsThe names of individual data tiles refer to the longitude and latitude of the lower-left (southwest) corner of the tile (this follows the DTED convention as opposed to the GTOPO30 standard). For example, the coordinates of the lower-left corner of tile N40W118 are 40 degrees north latitude and 118 degrees west longitude. To be more exact, these coordinates refer to the geometric center of the lower left pixel, which in the case of SRTM-1 data will be about 30 meters in extent.SRTM-1 data are sampled at one arc-second of latitude and longitude andeach file contains 3601 lines and 3601 samples. The rows at the north and south ecges as well as the columns at the east and west edges of each cell overlap and are identical to the edge rows and columns in the adjacent cell."and"3.1 DEM File (.HGT)The DEM is provided as 16-bit signed integer data in a simple binary raster. There are no header or trailer bytes embedded in the file. The data are stored in row major order (all the data for row 1, followed by all the data for row 2, etc.).All elevations are in meters referenced to the WGS84 geoid. Note that this from data processed by the "PI Processor", which uses the WGS84 ellipsoid.Byte order is Motorola ("big-endian") standard with the most significant byte first. Since they are signed integers elevations can range from -32767 to 32767 meters, encompassing the range of elevation to be found on the Earth.In these preliminary data there commonly will be data voids from a number of causes such as shadowing, phase unwrapping anomalies, or other radar-specific causes. Voids are flagged with the value -32768."and "4.3 SRTM CaveatsNo editing has been performed on the data, and the elevation data in particular contain numerous voids and other spurious points such as anomalously high (spike) or low (well) values. Water bodies will generally not be well-defined - in fact since water surfaces generally produce very low radar backscatter they will appear quite "noisy" or rough, in the elevations data. Similarly, coastlines will not be well-defined. No editing has been performed on the data, and the elevation data in particular contain numerous voids and other spurious points such as anomalously high (spike) or low (well) values. Water bodies will generally not be well-defined - in fact since water surfaces generally produce very low radar backscatter they will appear quite "noisy" or rough, in the elevations data. Similarly, coastlines will not be well-defined."Cheers, Holger

[Guest roto2]

>When I d/l let's say S18W068.HGT.ZIP (which contains the file>S18W068.HGT) which area is then covered by these data. I know,>S18-W068 is one of the corners of a one degree long./lat.>rectangle. But streches it from S18 to S19 or to S17. And W068>to W069 or W067?S18W068.HGT => Latitude S17 to S18, and longitude W068 to W067.>Does it make any difference for north/south and east/west>hemisphere?>I mean, when S18W068 is e.g. the lower left corner, is it then>always the lower left corner no matter which hemisphere?>No difference.Always lower left corner.

[Guest Jozef_K]

Thanks Holger and Roto2 for your answers."Always lower left corner". and"S18W068.HGT => Latitude S17 to S18, and longitude W068 to W067."There is a contradiction in it.From Latitude S17 to S18 is going south (down). In that case S18W068 would be the left UPPER corner. But I assume that "to S18" is a typo and you mean "to S16 (which is "up/north".Jozef http://www.dse.nl/~joker32/pictures/signature.jpg

[Guest roto2]

>"Always lower left corner". >and>"S18W068.HGT => Latitude S17 to S18, and longitude W068 to>W067.">There is a contradiction in it.>>From Latitude S17 to S18 is going south (down). In that case>S18W068 would be the left UPPER corner. But I assume that "to>S18" is a typo and you mean "to S16 (which is "up/north".Everything is relative. :DI think this will explain better:File S18W068.HGT:S17 W068 --------- S17 W067|.......................................||.......................................||.......................................||.......................................||.......................................|S18 W068 --------- S18 W067

[Guest Jozef_K]

Roto2 wrote: "I think this will explain better:"Absolutely! The figure shows it perfectly. Thanks.Jozef http://www.dse.nl/~joker32/pictures/signature.jpg

[Guest]

Hi!The small sketch is absolutely right. However, a one degree HGT quadrangle does not deliver a one degree Mesh Terrain BGL file. The area actually covered on the ground depends on the LOD factor you choose.I discover that when I made two Mesh terrain for the highest mountains in Brazil.To avoid "blank areas", you will have to make full use of the MultiSource option of the INF files, and work with several DEM one degree quadrangles (generated from correspondant HGT files).

[Guest]

>The area actually covered on the ground depends on the>LOD factor you choose.>Any one care to explain to a newbe??>>To avoid "blank areas", you will have to make full use of the>MultiSource option of the INF files, and work with several DEM>one degree quadrangles >the same here!?!? or just point me to a place vere I could get the info.Cheers!!Adj from Sweden

[Guest]

More involved explanation on Elevated Mesh Terrain:LOD factor: Level Of Detail (LOD) defines two things. The size and position of the Elevated Mesh Tiles, which cover the entire Earth with very different Tile sizes, and the sample cell size.Examples:LOD8 is made of Tiles with 0.3515625 x 0.46875 degrees each(Lat.xLong.). The sample cell size is 152.9m. That is, every 152.9m you get a point with an elevation value.LOD9 is made of Tiles with 0.17578125 x 0.234375 degrees each(Lat.xLong.). The sample cell size is 76.4m.If you increase the LOD by one unit, the correspondent Tiles will have one quarter of the original area.The best LOD for SRTM data is LOD9.Default scenery in FS2002 has just LOD5 in most of the regions: one elevation value every 1228 meters! You can miss an entire Hill with that.Multisource option: use this instead of the "use source dimensions" option, to control the way the Resample program builds the Tiles. You should edit the INF file manually, however. The FSTerrain software creates default INF files that you can edit in such a way the multisource option will be used. Read the documents in the MS Terrain SDK for complete information on this option.To make things clear, I will try to post the INF file I have used in one of the Mesh Sceneries I create.

[Guest]

>LOD8 is made of Tiles with 0.3515625 x 0.46875 degrees>each(Lat.xLong.). The sample cell size is 152.9m. >LOD9 is made of Tiles with 0.17578125 x 0.234375 degrees>each(Lat.xLong.). The sample cell size is 76.4m.>How do I calculate the right LOD?My indata have a cell size of 50 meters>>>To make things clear, I will try to post the INF file I have>used in one of the Mesh Sceneries I create.>Tanx a lot it gives some clues!And tank you for your help!!cheers m8!!!