[GRASS5] GRASS raster "live links" proposal
neteler at itc.it
Wed Feb 20 12:15:52 EST 2002
Proposal for linking external raster data into GRASS mapsets through GDAL
Nowadays GIS raster data can require a tremendous amount of harddrive space.
Especially in image processing of high resolution data hardware limitations
are reached quickly. The current implementation of GRASS 5.0 requires to
import raster data into a LOCATION before allowing for spatial analysis.
This leads to a replication of the data which is often not desired.
Our proposal is to modify the GRASS 5 raster library to additionally enable
GRASS to read from external raster files directly. Like that data can be
shared with other applications such as MapServer etc. The data might even be
shared over the network (by NFS for instance) but still appear within
your grass database as part of a mapset.
The idea is to implement read routines based on GDAL to enable GRASS to
read from raster files linked or copied into a LOCATION/MAPSET. The
'cellhd/' might contain a pathname to the raster file as an extra line
and this could be the "clue" to the raster reading code to use GDAL.
We would imagine something like r.link.gdal (as opposed to r.in.gdal) being
used to setup the virtual raster map without actually copying any raster
data and thereafter it would appear as a first class raster to all other
GRASS tools. The implementation might be a conditionally compiled in option
Database File Changes
The metadata (color table, cellhd info, null and range info) would all
remain in the existing file types within the GRASS mapset. However the
actual raster data (normally found in the cell/<name> or fcell/<name>
directories within the mapset would be missing in the case of linked
As well, the cellhd file would have an additional "link_filename:" line
containing the file containing the actual raster file being linked to, and
a "link_band:" line indicating what band within the referred to file was
linked to. These extra lines would only occure in "linked" cell headers,
not regular local ones.
Because virtually all of the metadata is stored in the regular auxilary
files, it is anticipated that not very many functions would need to be
altered. In particular the following (or underlying internal functions)
would need to be altered:
It is anticipated that the changes to support GDAL would be conditionally
compiled into libgis and that configure options would be supplied to
determine if linking is to be compiled in.
Target Code Base:
It is intended that this change would be developed in the GRASS 5.1 tree.
The r.link.gdal (or perhaps just r.link?) program would be added. It would
operate similarly to r.in.gdal but instead of actually copying the raster
data it would setup a link. The rest of the program operation would be the
same as r.in.gdal including support for image groups, GCPs and so forth.
In fact, it might be best to add a "link" option to r.in.gdal, with r.link
possibly being a shell script over r.in.gdal.
It might be necessary to modify this slightly to remove linked cells properly
(though it shouldn't delete the file linked to).
Modify to list linked filename and band for linked datasets.
Potentially a new program will be needed to update the link pointer if the
external file moves. Should this program also support "refreshing" metadata
with regard to raster updates to the external file? I am suggesting
updating the null mask, range info and histogram info.
o Should we really put the link information in the cellhd file? I don't
think it is desirable to extend the Cell_Head structure, so perhaps we
should keep the link information somewhere else?
o The G_open_cell_old() code for linked rasters would need to do some
consistency checking to ensure that the GDAL raster size still matches
that in the cellhd file. Should we try to recognise when the raster file
has changed, and that information about the linked file in GRASS, such as
the histogram, range and nulls may be wrong? I think we should not but
this may cause problems if the linked file is altered outside of GRASS.
o Should we try to use the GDAL file's native concept of nulls instead of
that in GRASS? For now I think not, but perhaps eventually.
o At the code level GRASS already supports virtual datasets defined by
r.mapcalc, right? Perhaps the code that supports this will provide a
guide for where to hook in the GDAL support code.
o Are the code segments that maintain range information, histograms and so
forth abstracted from the raster data access? If they are built right
into the code that writes the cells there could be problems with linked
files and extra work to be done. If they build up their meta information
using the "regular" raster access API there should be no problems.
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