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| Subject: | RE: GISList: Working with large rasters ~100Gb and GIS |
| Date: |
03/17/2003 10:17:19 AM |
| From: |
Voets, D. |
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Dear Bruce,
MrSID and ECW do not alter your resolution in any way. There is no geometric
distortion in any way. The geometric values of the pixels might change, but
that is all. In the conversion software from TIF to MrSID or to ECW, you are
free to choose a level of compression. A higher level of compression will
give you a smaller file, but the quality of the file wil degrade
subsequently. As a rule of thumb, you might say that a compression ration of
20:1 doesn't degrade the image in any way, at least according to the human
eye. Unless you do some radiometric analysis on this data, you do not have
to fear a lossy compression in any way. That is to say, you would not have
to fear the modern compression techniques in ECW, MrSID and JPEG2000.
ArcSDE will basically be able to load all that data, but it will put
tremendous constraints on the database you use. This may not be the easiest
way to handle your data. There aren't too many databases and servers that
can handle this. Your data would probably double or triple, because of
indexes and storage procedures in the database.
On a side note, the TIF file format only allows you to store 2GB of data in
one file. A 100GB TIF-file is simply not an option.
So, your options would be:
- buy yourself a tremendous server, a tremendous database and a tremendous
amount of time, and ArcSDE will do the job for you. I don't think that's a
path that will make you happy. Beware though, it is possible.
- You might create about 50 TIF files of 2GB each. Or 100 TIF files of 1GB
each. That will give you a hard time administrating the data. Your network
traffic will go sky high. I don't think this will give you a great time
either.
- You might choose for a lossy compression, a modern one. At a compression
ratio of 20:1 (or 25:1, or 30:1, you should experiment with that), you
probably won't be able to see the difference, but your administration
problems will more or less diminish. Imagine that 25:1 would do, that would
give you a 4GB file for all your data. You might choose to split it into for
instance eight parts. That would give you the opportunity to put the files
on CD.
I'd say that going for MrSID or ECW would be the best thing to do in this
case.
Please let us all know what you end up with, and how you like the result.
All the best,
Dirk Voets
Support Engineer
ESRI Nederland B.V.
Tel : 010 - 217 07 50
Email : GISsupport@esrinl.com
Internet : http://www.esrinl.com
Log a call through the Internet:
http://www.esrinl.com/ondersteuning/callUK.asp
-----Original Message-----
From: Bannerman Bruce [mailto:BRUBAN@melbourne.vic.gov.au]
Sent: Friday, March 07, 2003 6:08 AM
To: 'gislist@geocomm.com': 'IMAGRS-L@CESNET.CZ'
Subject: GISList: Working with large rasters ~100Gb and GIS
Hello everyone,
Apologies for cross-posting.
I'm interested in finding real world experiences when working with large
raster images of approximately 100 Gb.
I'm not interested in claims by vendors.
We are about to receive a series of raster images (24bit, TIF 6 format).
These images have been derived from aerial photography and are >0.1m
resolution.
When combined they will form an orthophoto mosaic that is ~100 Gb.
*Ideally*, I'd like to manage this data as a single image for ease of use by
novice users and for data management purposes.
As we have business requirements that demand high resolution, I'd prefer not
to compress the image with a lossy format such as ECW or MrSid.
Our GIS environment is ESRI based. ArcGIS 8.3, ArcSDE 8.3, ArcIMS 4.0.1.
Novice GIS users have access to corporate spatial data via a browser. We
have no intention of adopting an alternate GIS.
Our preferred solution is to manage the raster data via ArcSDE.
Unfortunately, I'm finding it difficult to find an organisation that has
experience working with data similar to ours.
An alternate solution if ArcSDE proves not to be viable is to use ERMapper's
Image Web Server, despite the data loss issues.
Does anyone have any real world experiences when working with large ~100 Gb
raster data and GIS that they would be willing to share?
In particular I'm interested in response times experienced by end users.
Any thoughts would be appreciated.
Regards,
Bruce Bannerman
GIS Strategy Officer
City of Melbourne
Australia
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