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I did it just for grids, but the idea could be extended to intersection
of rectangles and polygons, as David suggests. Don't know how difficult
this would be for vector maps, but for raster maps the idea looks
viable to me. Dot-density maps *would* be useful for general mapping,
anyway.<br>
<br>
Jan<br>
<br>
On 05/06/10 19:51, David William Bitner wrote:
<blockquote
cite="mid:k2nd2f53e191005061051mac804d44i5ea17a3cae758cbd@mail.gmail.com"
type="cite">If it was the overall density of the dots that mattered
and not as much the exact number of dots, a representative table could
be made with a "random yet even" set of dots covering a certain area.
To apply to a polygon, you could then Scale the data to the appropriate
density and translate/compute the intersection to apply to a given
polygon.<br>
<br>
<div class="gmail_quote">On Thu, May 6, 2010 at 12:50 PM, Martin
Davis <span dir="ltr"><<a moz-do-not-send="true"
href="mailto:mbdavis@refractions.net">mbdavis@refractions.net</a>></span>
wrote:<br>
<blockquote class="gmail_quote"
style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">Did
your algorithm work with irregular polgon extents?
<div>
<div class="h5"><br>
<br>
Jan Hartmann wrote:<br>
<blockquote class="gmail_quote"
style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
I did this long ago. Regular points won't work, you'll get all kinds of
moire patterns. I just computed random x and y values from the origin
of the grid cell within the x- and y grid-size. Can't remember having
any problems with overlapping points.<br>
<br>
Jan<br>
<br>
On 05/06/10 19:35, Paul Ramsey wrote:<br>
<blockquote class="gmail_quote"
style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
Even-yet-random :) nice requirement. How about just starting with a<br>
regular grid and then perturbing the elements randomly with a radius<br>
of a cell size? You can use the area of the polygon and number of<br>
needed points to calculate the appropriate cell size and go from<br>
there.<br>
<br>
P<br>
<br>
On Thu, May 6, 2010 at 10:28 AM, Martin Davis<<a
moz-do-not-send="true" href="mailto:mbdavis@refractions.net"
target="_blank">mbdavis@refractions.net</a>> wrote:<br>
<br>
<blockquote class="gmail_quote"
style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
Good point about the need for even distribution of the points. That
seems<br>
like a whole lot harder to code than simply randomly placing points in a<br>
polygon. Does anyone have any pointers to algorithms for producing this<br>
effect?<br>
<br>
George Silva wrote:<br>
<br>
<blockquote class="gmail_quote"
style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
The really big problem with dot density is that dots can overlap<br>
themselves,<br>
masking the real number, so if anything will be developed in this area,<br>
the<br>
points should be<br>
<br>
A) evenly distributed<br>
or<br>
B) randomly distributed, but with some sort of "colision" tests, so
there<br>
is<br>
no or little overlap.<br>
<br>
This is a interesting idea, especially if we could make a materialized<br>
view<br>
with those points, which could be added to GIS software for
presentation.<br>
<br>
George<br>
<br>
On Thu, May 6, 2010 at 1:53 PM, Sufficool, Stanley<<br>
<a moz-do-not-send="true"
href="mailto:ssufficool@rov.sbcounty.gov" target="_blank">ssufficool@rov.sbcounty.gov</a>>
wrote:<br>
<br>
<br>
<br>
<blockquote class="gmail_quote"
style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
Looks nasty, but it might work:<br>
<br>
select<br>
st_line_interpolate_point(<br>
st_intersection(<br>
the_geom,<br>
st_makeline(<br>
st_pointn(st_exteriorring(the_geom), (rand1.rand *<br>
st_npoints(st_exteriorring(the_geom)))::int),<br>
st_pointn(st_exteriorring(the_geom), (rand2.rand *<br>
st_npoints(st_exteriorring(the_geom)))::int)<br>
)<br>
)<br>
,rand3.rand<br>
)<br>
from insert_your_table_name_here,<br>
(select random() as rand, generate_series(1,1000) as point_number) as<br>
rand1<br>
JOIN (select random() as rand, generate_series(1,1000) as point_number)<br>
as<br>
rand2<br>
ON rand1.point_number = rand2.point_number<br>
JOIN (select random() as rand, generate_series(1,1000) as point_number)<br>
as<br>
rand3<br>
ON rand2.point_number = rand3.point_number<br>
WHERE st_geometrytype(<br>
st_intersection(<br>
the_geom,<br>
st_makeline(<br>
st_pointn(st_exteriorring(the_geom), (rand1.rand *<br>
st_npoints(st_exteriorring(the_geom)))::int),<br>
st_pointn(st_exteriorring(the_geom), (rand2.rand *<br>
st_npoints(st_exteriorring(the_geom)))::int)<br>
)<br>
)<br>
) = 'ST_LineString'<br>
AND oid = 5030 /* Enter your own OID here */<br>
limit 100<br>
<br>
<br>
<br>
<br>
<br>
<blockquote class="gmail_quote"
style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
-----Original Message-----<br>
From: <a moz-do-not-send="true"
href="mailto:postgis-users-bounces@postgis.refractions.net"
target="_blank">postgis-users-bounces@postgis.refractions.net</a><br>
[mailto:<a moz-do-not-send="true"
href="mailto:postgis-users-bounces@postgis.refractions.net"
target="_blank">postgis-users-bounces@postgis.refractions.net</a>] On<br>
Behalf Of Martin Davis<br>
Sent: Thursday, May 06, 2010 8:56 AM<br>
To: John Abraham; <a moz-do-not-send="true"
href="mailto:postgis-users@postgis.refractions.net" target="_blank">postgis-users@postgis.refractions.net</a>;
Martin Davis<br>
Subject: Re: [postgis-users] Dot Density idea<br>
<br>
<br>
I was thinking the same thing!<br>
<br>
strk wrote:<br>
<br>
<br>
<blockquote class="gmail_quote"
style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
ST_RandomPoinsOnSurface(geometry, numpoints) would be an interesting<br>
function indeed. Sounds like a good job for GEOS/JTS.<br>
<br>
--strk;<br>
<br>
On Mon, May 03, 2010 at 10:49:32PM -0600, John Abraham wrote:<br>
<br>
<br>
<br>
<blockquote class="gmail_quote"
style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
One of the things I miss about using ESRI's GIS is the<br>
<br>
<br>
</blockquote>
</blockquote>
ability to do<br>
<br>
<br>
<blockquote class="gmail_quote"
style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
<blockquote class="gmail_quote"
style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">dot-density
maps. Within a polygon, the number of dots is<br>
<br>
<br>
</blockquote>
</blockquote>
proportional to a value, and the dots are randomly placed. I<br>
find it useful to be able to present several data values at<br>
once (e.g. blue dots for population, red dots for employment).<br>
<br>
<br>
<blockquote class="gmail_quote"
style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
<blockquote class="gmail_quote"
style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">I
also find that it is a more intuitive way of scaling for<br>
<br>
<br>
</blockquote>
</blockquote>
zone size<br>
<br>
<br>
<blockquote class="gmail_quote"
style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
<blockquote class="gmail_quote"
style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">than
dividing the value by the area of the zone. That is,<br>
<br>
<br>
</blockquote>
</blockquote>
the count<br>
<br>
<br>
<blockquote class="gmail_quote"
style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
<blockquote class="gmail_quote"
style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">of
the dots represents the actual number, but the density<br>
<br>
<br>
</blockquote>
</blockquote>
of the dots<br>
<br>
<br>
<blockquote class="gmail_quote"
style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
<blockquote class="gmail_quote"
style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">represents
the density of the number. So I don't have to decide<br>
whether to divide the value by the area of the polygon to plot<br>
density: both the absolute number and the density are<br>
<br>
<br>
</blockquote>
</blockquote>
easily visible.<br>
<br>
<br>
<blockquote class="gmail_quote"
style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
<blockquote class="gmail_quote"
style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">Since
my open-source GIS viewing systems (mostly QGIS and<br>
<br>
<br>
</blockquote>
</blockquote>
Mapserver)<br>
<br>
<br>
<blockquote class="gmail_quote"
style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
<blockquote class="gmail_quote"
style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">won't
plot dot-density, I've done without.<br>
<br>
But today I realized that I can build these on the server<br>
<br>
<br>
</blockquote>
</blockquote>
instead. I<br>
<br>
<br>
<blockquote class="gmail_quote"
style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
<blockquote class="gmail_quote"
style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">can
generate random points within the bounding-box of the polygon,<br>
throwing out those that aren't contained within the polygon,<br>
repeating until I have enough. Then I can save these points as a<br>
separate layer, and display this layer using almost any desktop or<br>
web based viewer!<br>
<br>
Has anyone done this? Can I do it in SQL or do I need to write<br>
something in PL/pgsql?<br>
<br>
-- <br>
John Abraham<br>
<br>
PS I just bought the Postgis In Action book; enjoying it so far.<br>
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<br>
<br>
</blockquote>
<br>
<br>
</blockquote>
-- <br>
Martin Davis<br>
Senior Technical Architect<br>
Refractions Research, Inc.<br>
(250) 383-3022<br>
<br>
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-- <br>
Martin Davis<br>
Senior Technical Architect<br>
Refractions Research, Inc.<br>
(250) 383-3022<br>
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-- <br>
<div>
<div class="h5">Martin Davis<br>
Senior Technical Architect<br>
Refractions Research, Inc.<br>
(250) 383-3022<br>
<br>
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-- <br>
************************************<br>
David William Bitner<br>
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