[Gdal-dev] Re: Enhancement to access Arc/Info Binrary Grid in zip file

[elias]

Kevin Ruland <kruland <at> ku.edu> writes:

> 
> Frank,
> 
> Thanks much for the sanity check.  Random access writes would hurt.  
> About the only way for it to work effectively would be to unzip into 
> memory, modify, and rezip.
> 
> I was digging through the aigrid code and see that it manipulates file 
> pointers directly, so it would be a great effort to work it in even for 
> my prinicipal interest,  read-only access.
> 
> Kevin
> 
> Frank Warmerdam wrote:
> 
> > Kevin Ruland wrote:
> >
> >> I've been lurking for a little while and am a relatively new user of 
> >> the GDAL library.  Please excuse any "neubies" I might make.
> >>
> >> I'm in the process of cataloging all of our raster datasets and am 
> >> troubled by the Arc/Info multiple file setup.  Keeping track of 
> >> multiple files is significantly harder than tracking them all in, 
> >> say, a zip file.  I was wondering if anyone has considered storing 
> >> these multi-resource formats in a container of some kind, what the 
> >> implications are, and if it would even be worthwile to persue.
> >
> >
> >
> > Kevin,
> >
> > I don't think it would be particularly easy to implement this though 
> > it is in theory doable, at
> > least for one format at a time.
> >
> > One problem is that formats like Arc/Info binary coverages are random 
> > access, and in a zip
> > file each file is compressed as one big stream.  That means if you 
> > want data from the middle
> > of the file, you need to uncompress all the preceeding data (and 
> > discard it) to get the data
> > you want.  In some cases this could completely hammer performance.
> >
> > While I sympathaize with your concern about managability of directory 
> > trees, I am doubtful
> > it is practical from a work or a performance point of view to change 
> > most of the multi-file
> > formats to support zipped version.
> >
> > Best regards.
> >
> 


don't worry boys
i've advance to know more of the grid format.
what i've foud (a lot of month ago, for the problem of the trial licence) is 
that the raster file is loaded into the memory as a matrix

(x1,y1) -----------------------------  (x2,y1)
        |                            |
        |                            |
        |                            |
(x1,y2) ------------------------------ (x2,y2)


and the representation of the tiles and data tiles as follows

12345678901234567890123456789012345678901234567890
tile 0       tile 1         tile 2 ...      tile n
data         data           data            data
tile         tile           tile            tile
0            1              2               3

when the program write it to the disk, makes an optimization algotihm, in order 
to accelerate the load of the data and show faster in the monitor. the result 
is a "desorder" process of writting

the last thing that i could do is what i to open the raster file (even 
the "somehow values related to georeferencing": integer offset of x,y values, 
related to point (x1,y2).

just when i started to work in write raster files, the license ends... 

anyway, if you know how to make functional a trial license of arcgis 8.3 please 
let me know

bye

Elias