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1.2 precision (II)

From: Greg Sepesi <sepesi@eduneer.com>
Date: Fri, 06 Feb 2004 18:00:31 -0500
Message-ID: <40241C8F.BD500FC0@eduneer.com>
To: www-svg@w3.org

Allowing an offset, possibly specified by a viewBox, to be subtracted
from geographic coordinates before they are stored in the DOM's
single-precision floating point storage would result in better
coordinate resolution, without giving up the benefits of full geographic
coordinates in the text document or the storage efficiency of
single-precision floating points in the DOM.

- - -
Greg

-------- Original Message --------
Subject: 1.2 precision
Resent-Date: Fri,  6 Feb 2004 15:48:59 -0500 (EST)
Resent-From: www-svg@w3.org
Date: Fri, 06 Feb 2004 15:47:31 -0500
From: Greg Sepesi <sepesi@eduneer.com>
To: www-svg@w3.org


Although I saw no mention of precision in the SVG 1.2 working draft,
section 4.1 (Basic data types) of the SVG 1.1 specification states,

"a <number> has the capacity for at least a single-precision floating
point number (see [ICC32]) and has a range (at a minimum) of -3.4e+38F
to +3.4e+38F.  It is recommended that higher precision floating point
storage and computation be performed on operations such as coordinate
system transformations to provide the best possible precision and to
prevent round-off errors."

A discussion of the advantages of using geographic coordinates is in SVG
1.1 section 7.12 (Geographic Coordinate Systems).  Storing longitude in
single-precision floating point format results in a resolution of 2.4
meters (i.e., (180 degrees longitude)(60 minutes/degree)(1852
meters/minute)/2^23), which is better than the accuracy of most consumer
GPS receivers presently available.  

However given that

- the use of full geographic coordinates simplifies the sharing/mining
of geographical information,
- consumer GPS receiver accuracy continues to improve (and more consumer
GPS receiver manufacturers are offering post-processing software to
obtain sub-meter accuracy), 
- surveillance GPS receivers already offer centimeter accuracy,
- there are many small scale mapping applications that would benefit
from more resolution (e.g., a 5 centimeter resolution map for a
botanical garden can show the relative location of individual specimens)
even though the available location accuracy might be significantly less 

it would be useful to be able to specify the precision of the
coordinates stored in the DOM, allowing double-precision for documents
that would benefit from the added precision.

- - -
Greg
Received on Friday, 6 February 2004 18:01:30 GMT

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