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| GeoCommunity Mailing List |
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| Subject: | Re: [gislist] Free GIS Software |
| Date: |
05/05/2005 12:55:01 PM |
| From: |
Tripp C. Corbin |
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You should try the world of DWG files where a new version is released
every other year or so it seems.
Charles C. (Tripp) Corbin III, GISP, MCP
Associate Vice President, GIS/Mapping
ESRI Authorized Instructor
Keck & Wood, Inc.
www.keckwood.com
(678) 417-4013
(678) 417-8785 fax
Keck & Wood now offers Introduction to ArcGIS/ArcView training classes.
Contact me for more information.
Currently scheduled classes:
Introduction to ArcGIS I - May 19-20
Introduction to ArcGIS II - May 23-25
-----Original Message-----
From: Chris 'Xenon' Hanson [mailto:xenon@alphapixel.com]
Sent: Thursday, May 05, 2005 12:00 PM
To: gislist@lists.thinkburst.com
Subject: Re: [gislist] Free GIS Software
Frank Warmerdam wrote:
> The .shx file contains the offset of each shape in the .shp file. So,
> essentially you can think of it as an "index" built on the shapeid.
>
> Without it, if you wanted shape 1034 in abc.shp you would need to
> scan through from the beginning, counting as you process shapes,
> till you found the 1034th. With the .shx you can just lookup the
1034th
> entry in the .shx to get the file offset in the .shp to go to.
>
> The .shx is really only important for variable length shapes. So for
> point files it is unnecessary since all points are the same length.
> But for lines and polygons it helps alot for lookup-by-index.
Until recently, my software read SHP files without using the SHX,
simply by sequential
record reading. We didn't need random-access, so this was easy enough.
Also, we'd
encountered situations where a user had mistakenly omitted the SHX file
in transferring data.
A few weeks ago, this was blown out of the water by a SHP file I
received, allegedly
written by an ESRI product (never did establish the exact ancestry of
the file/data). In
this file, two records were separated (786 records in...) by some junk
padding data. The
SHX file correctly indicated that the 787th record began some distance
after where the
786th record ended.
Our sequential reader had no way to cope with the problem.
Thus, we added SHX support, and if the SHX file is missing, we fall
back to the
sequential reader.
The SHP world is a strange place indeed.
> Best regards,
--
Chris 'Xenon' Hanson, omo sanze lettere Xenon
AlphaPixel.com
PixelSense Landsat processing now available!
http://www.alphapixel.com/demos/ "There is no Truth. There is only
Perception. To Perceive is to Exist." - Xen
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