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Points within polygons in different projections
What kinds of line segments/edges require high accuracy in a true surface-of-the-ellipsoid representation?How to project the result of a QueryTask in ArcGIS Javascript API?Transformation of user-defined projected coordinatesystemsSome terms related to coordinatePartial Map Pixel ProjectionsCalculating closest distances in meters/kilometers given lat/long coordinates?Overlay polygons on raster in different projectionsvoronoi / thiessen different CRSProblems transforming polygon projections inside Shiny appHow to determine if lat/long is within SABS school boundary?
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My silly doubt of the day is the following.
GIVEN. A polygon and a point feature classes in some projection. The points are contained within the polygons
QUESTION. The points are always inside the polygons no matter the projection destination I'm projecting points and polygons to ? (including 'unprojecting' to geographic)
coordinate-system reprojection-mathematics
edited 7 hours ago
Vince
15k33050
asked 8 hours ago
SupereshekSupereshek
61
New contributor
Supereshek is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
add a comment |
My silly doubt of the day is the following.
GIVEN. A polygon and a point feature classes in some projection. The points are contained within the polygons
QUESTION. The points are always inside the polygons no matter the projection destination I'm projecting points and polygons to ? (including 'unprojecting' to geographic)
coordinate-system reprojection-mathematics
edited 7 hours ago
Vince
15k33050
asked 8 hours ago
SupereshekSupereshek
61
New contributor
Supereshek is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
add a comment |
My silly doubt of the day is the following.
GIVEN. A polygon and a point feature classes in some projection. The points are contained within the polygons
QUESTION. The points are always inside the polygons no matter the projection destination I'm projecting points and polygons to ? (including 'unprojecting' to geographic)
coordinate-system reprojection-mathematics
edited 7 hours ago
Vince
15k33050
asked 8 hours ago
SupereshekSupereshek
61
New contributor
Supereshek is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
My silly doubt of the day is the following.
GIVEN. A polygon and a point feature classes in some projection. The points are contained within the polygons
QUESTION. The points are always inside the polygons no matter the projection destination I'm projecting points and polygons to ? (including 'unprojecting' to geographic)
coordinate-system reprojection-mathematics
coordinate-system reprojection-mathematics
edited 7 hours ago
Vince
15k33050
asked 8 hours ago
SupereshekSupereshek
61
New contributor
Supereshek is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
edited 7 hours ago
Vince
15k33050
asked 8 hours ago
SupereshekSupereshek
61
New contributor
Supereshek is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
edited 7 hours ago
Vince
15k33050
edited 7 hours ago
Vince
15k33050
edited 7 hours ago
Vince
15k33050
15k33050
asked 8 hours ago
SupereshekSupereshek
61
New contributor
Supereshek is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
asked 8 hours ago
SupereshekSupereshek
61
asked 8 hours ago
SupereshekSupereshek
61
61
New contributor
Supereshek is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
New contributor
Supereshek is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
add a comment |
add a comment |
2 Answers
2
active
oldest
votes
Some software will use great circle arcs to connect unprojected vertices (sometimes when using a special data type, like PostGIS geography), while projected (or unprojected using the geometry datatype) vertices are connected using straight lines.
This can result in a point being inside a polygon expressed as geography but outside of it if expressed as geometry
The following example uses PostGIS. The polygon goes up to latitude 50, the point is at latitude 51.
WITH poly AS (select ST_GeomFromText('polygon((0 0, 50 0, 50 50, 0 50, 0 0))',4326) as geom),
pnt AS (select ST_GeomFromText('point(25 51)',4326) as geom)
SELECT ST_INTERSECTS(poly.geom,pnt.geom) intersect_geometry,
ST_INTERSECTS(poly.geom::geography,pnt.geom::geography) intersect_geography
FROM poly, pnt;
intersect_geometry | intersect_geography
--------------------+---------------------
f | t
answered 7 hours ago
JGHJGH
14.3k21439
add a comment |
Yes. projections will never reproject a point that was inside a polygon in one projection to be outside it in another--unless there's some sort of precision error. I'm not sure what this property is called in geography, but I just realized it's essentially relativistic invariance, which basically says that as time dilates for us, and our coordinate systems are compressed or stretched, no observer in any given frame will disagree on causal ordering of events. Likewise, no matter in what projection an "observer lives", no one will disagree on what points are in what polygons, even if they disagree on exactly how far the points are from the edges of the polygons, and stuff like that.
answered 8 hours ago
0mn10mn1
283
Well, except for horizon clipping, and discontinuous projections, which might throw a monkey wench in topology.
– Vince
7 hours ago
3
Not sure but I think the question come from the way polygon may be incorrectly reprojected as in a big polygon defined by only corner vertex. When reprojecting you will project the corner then reconstruct the polygon by linking the corner with a straight line. In this case a point close to the border could be seen on the other side after reprojecting but that's because the projection is wrong along the length of the side (to prevent that you need to add vertex on the polygon side before reprojecting)
– J.R
7 hours ago
I hadn't considered horizons or discontinous projections. I guess like in physics geography does have singularities where certain space-time coordinate systems break down; poles are another example.
– 0mn1
5 hours ago
add a comment |
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
Some software will use great circle arcs to connect unprojected vertices (sometimes when using a special data type, like PostGIS geography), while projected (or unprojected using the geometry datatype) vertices are connected using straight lines.
This can result in a point being inside a polygon expressed as geography but outside of it if expressed as geometry
The following example uses PostGIS. The polygon goes up to latitude 50, the point is at latitude 51.
WITH poly AS (select ST_GeomFromText('polygon((0 0, 50 0, 50 50, 0 50, 0 0))',4326) as geom),
pnt AS (select ST_GeomFromText('point(25 51)',4326) as geom)
SELECT ST_INTERSECTS(poly.geom,pnt.geom) intersect_geometry,
ST_INTERSECTS(poly.geom::geography,pnt.geom::geography) intersect_geography
FROM poly, pnt;
intersect_geometry | intersect_geography
--------------------+---------------------
f | t
answered 7 hours ago
JGHJGH
14.3k21439
add a comment |
Some software will use great circle arcs to connect unprojected vertices (sometimes when using a special data type, like PostGIS geography), while projected (or unprojected using the geometry datatype) vertices are connected using straight lines.
This can result in a point being inside a polygon expressed as geography but outside of it if expressed as geometry
The following example uses PostGIS. The polygon goes up to latitude 50, the point is at latitude 51.
WITH poly AS (select ST_GeomFromText('polygon((0 0, 50 0, 50 50, 0 50, 0 0))',4326) as geom),
pnt AS (select ST_GeomFromText('point(25 51)',4326) as geom)
SELECT ST_INTERSECTS(poly.geom,pnt.geom) intersect_geometry,
ST_INTERSECTS(poly.geom::geography,pnt.geom::geography) intersect_geography
FROM poly, pnt;
intersect_geometry | intersect_geography
--------------------+---------------------
f | t
answered 7 hours ago
JGHJGH
14.3k21439
add a comment |
Some software will use great circle arcs to connect unprojected vertices (sometimes when using a special data type, like PostGIS geography), while projected (or unprojected using the geometry datatype) vertices are connected using straight lines.
This can result in a point being inside a polygon expressed as geography but outside of it if expressed as geometry
The following example uses PostGIS. The polygon goes up to latitude 50, the point is at latitude 51.
WITH poly AS (select ST_GeomFromText('polygon((0 0, 50 0, 50 50, 0 50, 0 0))',4326) as geom),
pnt AS (select ST_GeomFromText('point(25 51)',4326) as geom)
SELECT ST_INTERSECTS(poly.geom,pnt.geom) intersect_geometry,
ST_INTERSECTS(poly.geom::geography,pnt.geom::geography) intersect_geography
FROM poly, pnt;
intersect_geometry | intersect_geography
--------------------+---------------------
f | t
answered 7 hours ago
JGHJGH
14.3k21439
Some software will use great circle arcs to connect unprojected vertices (sometimes when using a special data type, like PostGIS geography), while projected (or unprojected using the geometry datatype) vertices are connected using straight lines.
This can result in a point being inside a polygon expressed as geography but outside of it if expressed as geometry
The following example uses PostGIS. The polygon goes up to latitude 50, the point is at latitude 51.
WITH poly AS (select ST_GeomFromText('polygon((0 0, 50 0, 50 50, 0 50, 0 0))',4326) as geom),
pnt AS (select ST_GeomFromText('point(25 51)',4326) as geom)
SELECT ST_INTERSECTS(poly.geom,pnt.geom) intersect_geometry,
ST_INTERSECTS(poly.geom::geography,pnt.geom::geography) intersect_geography
FROM poly, pnt;
intersect_geometry | intersect_geography
--------------------+---------------------
f | t
answered 7 hours ago
JGHJGH
14.3k21439
answered 7 hours ago
JGHJGH
14.3k21439
answered 7 hours ago
JGHJGH
14.3k21439
answered 7 hours ago
JGHJGH
14.3k21439
14.3k21439
add a comment |
add a comment |
Yes. projections will never reproject a point that was inside a polygon in one projection to be outside it in another--unless there's some sort of precision error. I'm not sure what this property is called in geography, but I just realized it's essentially relativistic invariance, which basically says that as time dilates for us, and our coordinate systems are compressed or stretched, no observer in any given frame will disagree on causal ordering of events. Likewise, no matter in what projection an "observer lives", no one will disagree on what points are in what polygons, even if they disagree on exactly how far the points are from the edges of the polygons, and stuff like that.
answered 8 hours ago
0mn10mn1
283
Well, except for horizon clipping, and discontinuous projections, which might throw a monkey wench in topology.
– Vince
7 hours ago
3
Not sure but I think the question come from the way polygon may be incorrectly reprojected as in a big polygon defined by only corner vertex. When reprojecting you will project the corner then reconstruct the polygon by linking the corner with a straight line. In this case a point close to the border could be seen on the other side after reprojecting but that's because the projection is wrong along the length of the side (to prevent that you need to add vertex on the polygon side before reprojecting)
– J.R
7 hours ago
I hadn't considered horizons or discontinous projections. I guess like in physics geography does have singularities where certain space-time coordinate systems break down; poles are another example.
– 0mn1
5 hours ago
add a comment |
Yes. projections will never reproject a point that was inside a polygon in one projection to be outside it in another--unless there's some sort of precision error. I'm not sure what this property is called in geography, but I just realized it's essentially relativistic invariance, which basically says that as time dilates for us, and our coordinate systems are compressed or stretched, no observer in any given frame will disagree on causal ordering of events. Likewise, no matter in what projection an "observer lives", no one will disagree on what points are in what polygons, even if they disagree on exactly how far the points are from the edges of the polygons, and stuff like that.
answered 8 hours ago
0mn10mn1
283
Well, except for horizon clipping, and discontinuous projections, which might throw a monkey wench in topology.
– Vince
7 hours ago
3
Not sure but I think the question come from the way polygon may be incorrectly reprojected as in a big polygon defined by only corner vertex. When reprojecting you will project the corner then reconstruct the polygon by linking the corner with a straight line. In this case a point close to the border could be seen on the other side after reprojecting but that's because the projection is wrong along the length of the side (to prevent that you need to add vertex on the polygon side before reprojecting)
– J.R
7 hours ago
I hadn't considered horizons or discontinous projections. I guess like in physics geography does have singularities where certain space-time coordinate systems break down; poles are another example.
– 0mn1
5 hours ago
add a comment |
Yes. projections will never reproject a point that was inside a polygon in one projection to be outside it in another--unless there's some sort of precision error. I'm not sure what this property is called in geography, but I just realized it's essentially relativistic invariance, which basically says that as time dilates for us, and our coordinate systems are compressed or stretched, no observer in any given frame will disagree on causal ordering of events. Likewise, no matter in what projection an "observer lives", no one will disagree on what points are in what polygons, even if they disagree on exactly how far the points are from the edges of the polygons, and stuff like that.
answered 8 hours ago
0mn10mn1
283
Yes. projections will never reproject a point that was inside a polygon in one projection to be outside it in another--unless there's some sort of precision error. I'm not sure what this property is called in geography, but I just realized it's essentially relativistic invariance, which basically says that as time dilates for us, and our coordinate systems are compressed or stretched, no observer in any given frame will disagree on causal ordering of events. Likewise, no matter in what projection an "observer lives", no one will disagree on what points are in what polygons, even if they disagree on exactly how far the points are from the edges of the polygons, and stuff like that.
answered 8 hours ago
0mn10mn1
283
answered 8 hours ago
0mn10mn1
283
answered 8 hours ago
0mn10mn1
283
answered 8 hours ago
0mn10mn1
283
283
Well, except for horizon clipping, and discontinuous projections, which might throw a monkey wench in topology.
– Vince
7 hours ago
3
Not sure but I think the question come from the way polygon may be incorrectly reprojected as in a big polygon defined by only corner vertex. When reprojecting you will project the corner then reconstruct the polygon by linking the corner with a straight line. In this case a point close to the border could be seen on the other side after reprojecting but that's because the projection is wrong along the length of the side (to prevent that you need to add vertex on the polygon side before reprojecting)
– J.R
7 hours ago
I hadn't considered horizons or discontinous projections. I guess like in physics geography does have singularities where certain space-time coordinate systems break down; poles are another example.
– 0mn1
5 hours ago
add a comment |
Well, except for horizon clipping, and discontinuous projections, which might throw a monkey wench in topology.
– Vince
7 hours ago
3
Not sure but I think the question come from the way polygon may be incorrectly reprojected as in a big polygon defined by only corner vertex. When reprojecting you will project the corner then reconstruct the polygon by linking the corner with a straight line. In this case a point close to the border could be seen on the other side after reprojecting but that's because the projection is wrong along the length of the side (to prevent that you need to add vertex on the polygon side before reprojecting)
– J.R
7 hours ago
I hadn't considered horizons or discontinous projections. I guess like in physics geography does have singularities where certain space-time coordinate systems break down; poles are another example.
– 0mn1
5 hours ago
Well, except for horizon clipping, and discontinuous projections, which might throw a monkey wench in topology.
– Vince
7 hours ago
Well, except for horizon clipping, and discontinuous projections, which might throw a monkey wench in topology.
– Vince
7 hours ago
3
3
Not sure but I think the question come from the way polygon may be incorrectly reprojected as in a big polygon defined by only corner vertex. When reprojecting you will project the corner then reconstruct the polygon by linking the corner with a straight line. In this case a point close to the border could be seen on the other side after reprojecting but that's because the projection is wrong along the length of the side (to prevent that you need to add vertex on the polygon side before reprojecting)
– J.R
7 hours ago
Not sure but I think the question come from the way polygon may be incorrectly reprojected as in a big polygon defined by only corner vertex. When reprojecting you will project the corner then reconstruct the polygon by linking the corner with a straight line. In this case a point close to the border could be seen on the other side after reprojecting but that's because the projection is wrong along the length of the side (to prevent that you need to add vertex on the polygon side before reprojecting)
– J.R
7 hours ago
I hadn't considered horizons or discontinous projections. I guess like in physics geography does have singularities where certain space-time coordinate systems break down; poles are another example.
– 0mn1
5 hours ago
I hadn't considered horizons or discontinous projections. I guess like in physics geography does have singularities where certain space-time coordinate systems break down; poles are another example.
– 0mn1
5 hours ago
add a comment |
Supereshek is a new contributor. Be nice, and check out our Code of Conduct.
Supereshek is a new contributor. Be nice, and check out our Code of Conduct.
Supereshek is a new contributor. Be nice, and check out our Code of Conduct.
Supereshek is a new contributor. Be nice, and check out our Code of Conduct.
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