Changes between Version 16 and Version 17 of WKTRaster/FAQ


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Timestamp:
Mar 22, 2010, 11:41:40 AM (14 years ago)
Author:
pracine
Comment:

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  • WKTRaster/FAQ

    v16 v17  
    33'''1 - What is the meaning of "WKT" in WKT Raster?'''
    44
    5 WKT stands for "[http://en.wikipedia.org/wiki/Well-known_text Well Known Text]". Is is an OGC standard to represent geographical information in a human readable textual representation. however, the OCG WKT standard does not define a way to represent raster yet. PostGIS WKT Raster is a bit ahead on this :-)
     5WKT stands for "[http://en.wikipedia.org/wiki/Well-known_text Well Known Text]". Is is an OGC standard to represent geographical information in a human readable textual representation. However, the OCG WKT standard does not define a way to represent raster yet. PostGIS WKT Raster is a bit ahead on this :-)
    66
    77'''2 - How is WKT Raster different than the Oracle SDO_GEORASTER and SDO_RASTER types?'''
    … …  
    1414 * rasters to overlaps (which is necessary to implement lossless vector to raster conversion)
    1515
    16 These arrangements are possible in Oracle as well, but they implies the storage of multiple SDO_GEORASTER objects linked to as many SDO_RASTER tables. A complex coverage can lead to hundreds of tables in the database. With WKT Raster you can store a similar raster arrangement into a unique table.
     16These arrangements are possible in Oracle as well, but they imply the storage of multiple SDO_GEORASTER objects linked to as many SDO_RASTER tables. A complex coverage can lead to hundreds of tables in the database. With WKT Raster you can store a similar raster arrangement into a unique table.
     17
     18It's a bit like if PostGIS would force you to store only full rectangular vector coverage without gaps or overlaps (a perfect rectangular topological layer). This is very practical in some applications but practice has shown that it is not realistic or desirable for most geographical coverages. Vector structures needs the flexibility to store discontinuous and non-rectangular coverages. We think it is a big advantage that raster structure should benefit as well.
    1719
    1820'''3 - Why do you say "WKT Raster goal is to implement the RASTER type as much as possible like the GEOMETRY type is implemented in PostGIS"? Vector and raster are very different and have always been threated differently in GIS packages.'''
    … …  
    2224'''4 - Why does every tile is georeferenced?'''
    2325
    24 In a traditional raster system only the global image is georeferenced, not every tile. The system assume that the area covered by the tiles forms a perfect rectangle well aligned on a grid. This is the raster view of geospatial data. However the reality is often different. People use raster data covering area for which the sum of the tiles do not necessarily result in a rectangle. This is the coverage view of raster geospatial data. Having one georeference per tile allow WKT Raster to store raster coverage which are not necessarily rectangular.
     26In a traditional raster system only the global image is georeferenced, not every tile. The system assumes that the area covered by the tiles forms a perfect rectangle well aligned on a grid. This is the raster view of geospatial data. However the reality is often different. People use raster data covering area for which the sum of the tiles does not necessarily result in a rectangle. This is the coverage view of raster geospatial data. Having one georeference per tile allow WKT Raster to store raster coverage which are not necessarily rectangular.
    2527
    26 People also often need to convert vector data to raster data in order to perform certain operations. If you convert a vector coverage containing gaps or overlaps to raster, you may result in way too much information in the first case (gaps are filled with data) or in a lost of information in the second case (overlaps are blended or only one polygon is converted). In WKT Raster each vector object convert to a single raster so that there is a one to one correspondence between the two. Having one georeference per tile (or raster) is very usefull to properly support this feature.
     28People also often need to convert vector data to raster data in order to perform certain operations. If you convert a vector coverage containing gaps or overlaps to raster, you may result in way too much information in the first case (gaps are filled with data) or in a lost of information in the second case (overlaps are blended or only one polygon is converted). In WKT Raster each vector object converts to a single raster so that there is a one to one correspondence between the two. Having one georeference per tile (or raster) is very usefull to properly support this feature.
    2729
    2830'''5 - How come it is possible to store overlapping rasters in the same table? In a raster, tiles should not overlaps.'''