[prev in list] [next in list] [prev in thread] [next in thread]
List: postgis-users
Subject: Re: [postgis-users] curve between geo points
From: Jim Walseth <jwalseth () tableau ! com>
Date: 2017-05-25 21:10:30
Message-ID: BN6PR03MB31086131F5DEB341D3720196C5FF0 () BN6PR03MB3108 ! namprd03 ! prod ! outlook ! com
[Download RAW message or body]
[Attachment #2 (text/plain)]
Thank you François this is just the information I was seeking.
-Jim
From: postgis-users [mailto:postgis-users-bounces@lists.osgeo.org] On Behalf Of \
François B
Sent: Tuesday, May 23, 2017 10:12 AM
To: postgis-users@lists.osgeo.org
Subject: Re: [postgis-users] curve between geo points
It depends on the type of your column. If it's standard "geometry", then it's a \
straight line in the 2D space defined by your coordinate system. If it's "geography", \
then it's a curve, aka geodesic or great circle.
From the manual:
4.2. PostGIS Geography Type
https://postgis.net/docs/using_postgis_dbmanagement.html#PostGIS_Geography
The basis for the PostGIS geometry type is a plane. The shortest path between two \
points on the plane is a straight line. That means calculations on geometries (areas, \
distances, lengths, intersections, etc) can be calculated using cartesian mathematics \
and straight line vectors.
The basis for the PostGIS geographic type is a sphere. The shortest path between two \
points on the sphere is a great circle arc. That means that calculations on \
geographies (areas, distances, lengths, intersections, etc) must be calculated on the \
sphere, using more complicated mathematics. For more accurate measurements, the \
calculations must take the actual spheroidal shape of the world into account, and the \
mathematics becomes very complicated indeed.
Note that many data formats (outside of PostGIS, e.g. Shapefile), do not explicitly \
define this interpretation. See this Stack Exchange question:
Are long lines in shapefiles to be considered geodesics or straight lines in the 2D \
latlong space? https://gis.stackexchange.com/q/25670/7599
I'd like to know how PostGIS interprets the curve between two points on the earth, in \
the context of spatial analysis. Ex, the NW and NE corners of the State of Colorado - \
is it a 'straight line' in some projection, or a geodetic curve or the intersection \
of a normal plane with the surface (as in MS Sequel). Possibly, the function called \
implies this (2D VS. 3D).
Redirecting to online documentation or discussion would be welcome.
Best regards,
Jim Walseth
[Attachment #3 (text/html)]
<html xmlns:v="urn:schemas-microsoft-com:vml" \
xmlns:o="urn:schemas-microsoft-com:office:office" \
xmlns:w="urn:schemas-microsoft-com:office:word" \
xmlns:m="http://schemas.microsoft.com/office/2004/12/omml" \
xmlns="http://www.w3.org/TR/REC-html40"> <head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<meta name="Generator" content="Microsoft Word 15 (filtered medium)">
<style><!--
/* Font Definitions */
@font-face
{font-family:"Cambria Math";
panose-1:2 4 5 3 5 4 6 3 2 4;}
@font-face
{font-family:Calibri;
panose-1:2 15 5 2 2 2 4 3 2 4;}
/* Style Definitions */
p.MsoNormal, li.MsoNormal, div.MsoNormal
{margin:0in;
margin-bottom:.0001pt;
font-size:12.0pt;
font-family:"Times New Roman",serif;}
a:link, span.MsoHyperlink
{mso-style-priority:99;
color:blue;
text-decoration:underline;}
a:visited, span.MsoHyperlinkFollowed
{mso-style-priority:99;
color:purple;
text-decoration:underline;}
span.EmailStyle17
{mso-style-type:personal;
font-family:"Calibri",sans-serif;
color:#1F497D;}
span.EmailStyle18
{mso-style-type:personal-compose;
font-family:"Calibri",sans-serif;
color:windowtext;}
.MsoChpDefault
{mso-style-type:export-only;
font-family:"Calibri",sans-serif;}
@page WordSection1
{size:8.5in 11.0in;
margin:1.0in 1.0in 1.0in 1.0in;}
div.WordSection1
{page:WordSection1;}
--></style><!--[if gte mso 9]><xml>
<o:shapedefaults v:ext="edit" spidmax="1026" />
</xml><![endif]--><!--[if gte mso 9]><xml>
<o:shapelayout v:ext="edit">
<o:idmap v:ext="edit" data="1" />
</o:shapelayout></xml><![endif]-->
</head>
<body lang="EN-US" link="blue" vlink="purple">
<div class="WordSection1">
<p class="MsoNormal"><span \
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Thank \
you François this is just the information I was seeking.<o:p></o:p></span></p> <p \
class="MsoNormal"><span \
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">-Jim<o:p></o:p></span></p>
<p class="MsoNormal"><span \
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><b><span \
style="font-size:11.0pt;font-family:"Calibri",sans-serif">From:</span></b><span \
style="font-size:11.0pt;font-family:"Calibri",sans-serif"> postgis-users \
[mailto:postgis-users-bounces@lists.osgeo.org] <b>On Behalf Of </b>François B<br>
<b>Sent:</b> Tuesday, May 23, 2017 10:12 AM<br>
<b>To:</b> postgis-users@lists.osgeo.org<br>
<b>Subject:</b> Re: [postgis-users] curve between geo points<o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
<div>
<div>
<p class="MsoNormal">It depends on the type of your column. If it's standard \
"geometry", then it's a straight line in the 2D space defined by your \
coordinate system. If it's "geography", then it's a curve, aka geodesic or \
great circle.<o:p></o:p></p> </div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">From the manual:<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">4.2. PostGIS Geography Type<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><a \
href="https://postgis.net/docs/using_postgis_dbmanagement.html#PostGIS_Geography">http \
s://postgis.net/docs/using_postgis_dbmanagement.html#PostGIS_Geography</a><o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<blockquote style="border:none;border-left:solid #CCCCCC 1.0pt;padding:0in 0in 0in \
6.0pt;margin-left:4.8pt;margin-right:0in"> <p class="MsoNormal">The basis for the \
PostGIS geometry type is a plane. The shortest path between two points on the plane \
is a straight line. That means calculations on geometries (areas, distances, lengths, \
intersections, etc) can be calculated using cartesian mathematics and straight line \
vectors.<o:p></o:p></p> </blockquote>
<blockquote style="border:none;border-left:solid #CCCCCC 1.0pt;padding:0in 0in 0in \
6.0pt;margin-left:4.8pt;margin-right:0in"> <p class="MsoNormal"><o:p> </o:p></p>
</blockquote>
<blockquote style="border:none;border-left:solid #CCCCCC 1.0pt;padding:0in 0in 0in \
6.0pt;margin-left:4.8pt;margin-right:0in"> <p class="MsoNormal">The basis for the \
PostGIS geographic type is a sphere. The shortest path between two points on the \
sphere is a great circle arc. That means that calculations on geographies (areas, \
distances, lengths, intersections, etc) must be calculated on the sphere, using more \
complicated mathematics. For more accurate measurements, the calculations must take \
the actual spheroidal shape of the world into account, and the mathematics becomes \
very complicated indeed.<o:p></o:p></p> </blockquote>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">Note that many data formats (outside of PostGIS, e.g. \
Shapefile), do not explicitly define this interpretation. See this Stack Exchange \
question:<o:p></o:p></p> </div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">Are long lines in shapefiles to be considered geodesics or \
straight lines in the 2D latlong space?<o:p></o:p></p> </div>
<div>
<p class="MsoNormal"><a \
href="https://gis.stackexchange.com/q/25670/7599">https://gis.stackexchange.com/q/25670/7599</a><o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<blockquote style="border:none;border-left:solid #CCCCCC 1.0pt;padding:0in 0in 0in \
6.0pt;margin-left:4.8pt;margin-right:0in"> <p class="MsoNormal">I'd like to know how \
PostGIS interprets the curve between two points on the earth, in the context of \
spatial analysis. Ex, the NW and NE corners of the State of Colorado - is it a \
'straight line' in some projection, or a geodetic curve or the intersection of a \
normal plane with the surface (as in MS Sequel). Possibly, the function called \
implies this (2D VS. 3D).<o:p></o:p></p> </blockquote>
<blockquote style="border:none;border-left:solid #CCCCCC 1.0pt;padding:0in 0in 0in \
6.0pt;margin-left:4.8pt;margin-right:0in"> <p class="MsoNormal"><o:p> </o:p></p>
</blockquote>
<blockquote style="border:none;border-left:solid #CCCCCC 1.0pt;padding:0in 0in 0in \
6.0pt;margin-left:4.8pt;margin-right:0in"> <p class="MsoNormal">Redirecting to online \
documentation or discussion would be welcome.<o:p></o:p></p> </blockquote>
<blockquote style="border:none;border-left:solid #CCCCCC 1.0pt;padding:0in 0in 0in \
6.0pt;margin-left:4.8pt;margin-right:0in"> <p class="MsoNormal"><o:p> </o:p></p>
</blockquote>
<blockquote style="border:none;border-left:solid #CCCCCC 1.0pt;padding:0in 0in 0in \
6.0pt;margin-left:4.8pt;margin-right:0in"> <p class="MsoNormal">Best \
regards,<o:p></o:p></p> </blockquote>
<blockquote style="border:none;border-left:solid #CCCCCC 1.0pt;padding:0in 0in 0in \
6.0pt;margin-left:4.8pt;margin-right:0in"> <p class="MsoNormal"><o:p> </o:p></p>
</blockquote>
<blockquote style="border:none;border-left:solid #CCCCCC 1.0pt;padding:0in 0in 0in \
6.0pt;margin-left:4.8pt;margin-right:0in"> <p class="MsoNormal">Jim \
Walseth<o:p></o:p></p> </blockquote>
<div>
<p class="MsoNormal"> <o:p></o:p></p>
</div>
</div>
</div>
</div>
</body>
</html>
[Attachment #4 (unknown)]
_______________________________________________
postgis-users mailing list
postgis-users@lists.osgeo.org
https://lists.osgeo.org/mailman/listinfo/postgis-users
[prev in list] [next in list] [prev in thread] [next in thread]
Configure |
About |
News |
Add a list |
Sponsored by KoreLogic