[Gdal-dev] GDAL over internet

[Frank Warmerdam]

On 5/17/05, Frédéric Trastour <support at geoimage.fr> wrote:
> Hello,
> 
> our 3D viewer is actually accessing geomatic data DEM/Textures/Vector Layers  using GDAL & OGR.
> I'm searching a solution to make this application able to access this kind of data over internet.
> 
> I think to two possible solutions :
> 
> 1/OGDI :
> The GDAL OGDI driver seems to allow to access data both locally and over internet ( using the gltpd server ).
> In the GDAL distribution I use ( the one included in the Openv_FW package ) i can access, for example,
> DTED Data using this driver. The problem is that the OGDI supported format list is quite small and doesn't
> include any generic format ( able to support both DEM and Gray/RGB/Lut based textures ) like GEOtiff.
> 
> The ideal thing should be a GDAL Based OGDI Driver for GDAL ! With that, any format accessible via GDAL
> should be used over internet.

Frédéric,

In fact, there is an OGDI driver based on GDAL that exists, but it has
not been properly polished.  To be honest, I am not exactly sure where
the code is any more. 

However, I think the bigger barrier to using OGDI for this purpose is that
OGDI returns images with zlib compression.   This lossless compression
is very ineffectively for many kinds of images.  It might not be too bad for
dem data though. 

> I have found in the contrib/gdal section  of OGDI distribution, something called GDAL bridge.
> Is it related ?

Doh!  You are way ahead of me.  This is the OGDI GDAL driver.  It
uses a "gdal bridge" to dynamically load the GDAL library as well, but
that can be discarded, and the driver linked directly against GDAL. 
The "bridge" architecture isn't actively supported anymore.
 
> Does anyone really use the gltdp server ?

I for one have not made use of the gltpd server for a couple years.
 
> 2/ tsmApi
> I found on the web the following open source library : http://www.tvgeo.com/tsmApi/
> This library is dedicated to the access/delivery ( the delivery is done via an apache module )
> of imagery over internet using http.
> 
> A solution to my problem should be to develop a new GDAL driver based on the tsmApi.
> 
> Does anyone on this list already experiment with this library ?
> 
> I can't get it working perfectly. My apache server ( i recompile it, and recompile the tsm apache
> against the same version ) craches sometimes int the server module.
> 
> Thanks in advance for any hints or comments on this two solutions.
> Any other formats/driver/server possibilities should be OK.

I have no experience with the tsmApi. 

I would suggest a couple other options. 

 o A WMS/WCS based GDAL driver using http to fetch from 
   OGC WMS or WCS servers.  I have wanted to do this for a long
   time, but there are a few complications.  With appropriate file
   formats, you could get the data in lossy compressed forms like
   jpeg and jpeg2000 for respectible latency.  This also has the benefit
   that it is useful against a variety of public datasources in addition to
   servers you deploy yourself. 

 o Use "ecwp" or "jpip" protocols.  These are network protocols for
    access to remote ecw and jpeg2000 images and are intended to
    allow fast piecewise access to remote imagery datasets.  In theory
    the ECW and JP2KAK drivers are close to supporting these now.
    However, I have had hard to explain problems with both and I 
    haven't tried to promote them.  I'm not sure what is involved in 
    setting up ecwp or jpip servers though.  You may need to purchase
    commecial software to accomplish it. 

Best regards,

-- 
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