PostGIS Raster : Extending PostgreSQL for The Support of ... - CoDE

"32BUI" = 32-bit unsigned integer
"32BF" = 32-bit float
"64BF" = 64-bit float
4.9 Nodata Values
Cell values can be either positive or negative, integer or floating point. A noData value is used
to represent cell values that are either unknown or meaningless. In the other words, a noData value
represents the absence of data. Sometimes there are areas in a raster that user does not want to
display. These can include borders, backgrounds or other data considered to not have valid values. In
some cases, these areas are expressed as noData values, although in the other cases, they may have
real values.
When performing operations on raster data, there are typically two ways that noData are treated
for each cell: noData value is returned for the position of corresponding cell, noData is ignored and
can not be computed. So when calculating the statistics for a raster, user can decide to ignore or not
any cells with noData value.
Any analysis functions in PostGIS Raster always takes the nodata value into account. This means
that the areas with nodata value will be omitted from the operation. For example in intersection
operation, if a vector completely overlaps a nodata value area of one tile, the operation will set false
for this tile and vector intersection and returns an EMPTY GEOMETRY result [8]. If the vector partially
overlaps a nodata value area and intersect other parts of the tile, the operation will set true for this
intersection and returns only parts of the tile with significant values, omitting parts with nodata
values.
Each raster band has its own noData value whose range depends on the associated band pixel
type. The noData value is specified in the band information (see Figure ??).
4.10 Blocks or Tiles
When importing a raster image into PostGIS database, users can choose to split the raster into
small regular blocks by adding the -k option to the raster2pgsql.py Python loader, otherwise the
entire raster is loaded into a single block. The block size is a parameter that follows just after the
option -k (such as 128x128). All blocks are registered in a regular blocked table.
Each block is stored in a row of the table and contains all data about itself (such as attribute
information, georeference information, band information and band data) as the mother raster, except
its size is equal the size of mother raster divided by block size. So each block by definition can be
identified as a tile. This also means that in PostGIS Raster, there are no difference between rasters,
tiles and blocks.
Sometimes, the dimension size (row and column) of the mother raster may not be evenly divided
by the block size. PostGIST Raster adds padding to the boundary blocks that do not have enough
original cells to be completely filled. The padding cells have the same cell type as other cells and have
values equal to zero. Padding makes each block have the same block size.
Figure 4.28: Raster tiles [23].
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