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Yes, agreed, it would be good if this found it's way into GEOS and
PostGIS.<br>
<br>
(cc-ing geos-devel so they can get on it!)<br>
<br>
<br>
On 10/6/2011 11:42 PM, Peter Hopfgartner wrote:
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On 10/06/2011 06:33 PM, Martin Davis wrote:
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(...)<br>
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I suppose to provide even more accuracy the largest x value
(in absolute value) could be used as the constant offset.<br>
<br>
</blockquote>
From an heuristic POV I would guess, that the most accurate
result could be calculated, when the origin is placed in the
center of mass or center of the bounding box, since that would
keep the coordinates as small as possible.<br>
As I understand it, that would require to first calculate that
center estimate, which has at least the cost to loop through
all coordinates.<br>
From the rather synthetic test case, which I used, it is
difficult to have real numbers of the impact on the accuracy
of the result on that. If you think that there might be some
benefit, it might make sense to use real data, as in TIGER,
and compare the result obtained with floating points to the
result obtained with an arbitrary precision float library,
like <a moz-do-not-send="true" class="moz-txt-link-freetext"
href="http://www.apfloat.org">http://www.apfloat.org</a>.<br>
<br>
Having the origin in the first coordinate allows to drop the
first term, since x1 is zero.<br>
</blockquote>
As you say, it's probably not worth the effort to make this
minor improvement in accuracy.<br>
<br>
I had thought about using higher-precision computation to check
the area results. I would use the DD (DoubleDouble) class which
is now in JTS - it provides 106 bits of precision.<br>
<br>
And as you say, more real-world examples would be better for
testing. I might try and do this - although it doesn't seem
like anyone should be relying on the area computation to be
"perfectly" accurate. Or am I wrong on this? How did you come
to notice that the area was not as accurate as it could be?<br>
<br>
</blockquote>
I agree that nobody should rely on exact area calculations. We had
a minor problem, when we did things like splitting polygons in
PostGIS. We compared the area of the parts to the area to the
parent polygon ane the difference was bigger then expected. Having
a look at GEOS I saw that it used the same algorithm as PostGIS
(or vice versa) and from there I decided to start from JTS. Now I
like to apply tha same on GEOS and PostGIS.<br>
<br>
<a moz-do-not-send="true" class="moz-txt-link-freetext"
href="http://postgis.refractions.net/pipermail/postgis-devel/2011-February/011715.htm">http://postgis.refractions.net/pipermail/postgis-devel/2011-February/011715.htm</a>
was a first version of this patch.<br>
<br>
Peter<br>
<br>
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