Surely there is an “all of Norway” planar projection you can use to handle the integrative whole-country work cases? Even if there isn’t, you can always invent one and add it to the spatial_ref_sys table.
P > On Jun 7, 2019, at 3:02 AM, Lars Aksel Opsahl <[email protected]> wrote: > > Hi > > We have one issue/problem and that is how to handle Tolerance/SnapTo in a > good way when working in degrees. > > We work on maps that covers all of Norway so we use degrees > https://epsg.io/4258 <https://epsg.io/4258> . We also use Postgis Topology, > which means that we just have add new lines both for surface and line layers > and let Postgis Topology take care generating the surfaces so this samples I > just focus on lines. > > A description and some code pieces to illustrate the problem: > > A: Layers in meter > > First we show that this works well when working in meter. I create a Postgis > Topology layer with layer_precision 10 meter using https://epsg.io/25833 > <https://epsg.io/25833>. Then we add 4 horizontal lines and 4 vertical lines > using a precision with 2 meter. Postgis Topology handles this nicely and we > end with total of 4 lines and not 8 lines as we added, because every second > line snaps to the line added before because the distance between the lines > are less 2 meter which use as precision we used when adding new lines. > > The sample code > :https://github.com/NibioOpenSource/pgtopo_update_sql/blob/develop/src/test/sql/snapto/snapto_code_example_meter.sql > > <https://github.com/NibioOpenSource/pgtopo_update_sql/blob/develop/src/test/sql/snapto/snapto_code_example_meter.sql> > > A image of the result > :https://github.com/NibioOpenSource/pgtopo_update_sql/blob/develop/src/test/sql/snapto/snapto_code_example_meter.png > > <https://github.com/NibioOpenSource/pgtopo_update_sql/blob/develop/src/test/sql/snapto/snapto_code_example_meter.png> > > The green lines are the lines we added, the red lines are the lines that are > stored in the edge table. > > Layers in degrees : > > But when we are using degrees things starts be more difficult. The reason why > we are using degrees is to get a accurate transformations to local UTM zones > which are different depending on where in Norway you are. > > So the problem is how to use tolerances so we get a behavior equal to the > test using meter. > > We can we define the layer in Postgis Topology with quite big value because > this is just max value as it seems. So we can adjust the tolerance parameter > as we add lines but the problem is that we need to adjust this parameter > depending on where we are and what orientation the line has. For vertical > lines we need a bigger tolerance than for horizontal lines in Norway. This > makes it quite complicated to handle adding new lines. > > The sample code > :https://github.com/NibioOpenSource/pgtopo_update_sql/blob/develop/src/test/sql/snapto/snapto_code_example_degrees.sql > > <https://github.com/NibioOpenSource/pgtopo_update_sql/blob/develop/src/test/sql/snapto/snapto_code_example_degrees.sql> > > A image of the result > :https://github.com/NibioOpenSource/pgtopo_update_sql/blob/develop/src/test/sql/snapto/snapto_code_example_degrees.png > > <https://github.com/NibioOpenSource/pgtopo_update_sql/blob/develop/src/test/sql/snapto/snapto_code_example_degrees.png> > > > The green lines are the lines we added, the red lines are the lines that are > stored in the edge table. > > It real life it's impossible to do it this because water, farm land are not > built up by horizontal and vertical lines only. Since we use Postgis Topology > we could split a line in the different line parts and based on the angle > select the best tolerance value but that is complicated and means extra > computing. Another problem is that edges then will be broken up into > unnecessary small pieces. > > Any ideas on how to handle this in a better way ? > > Can “PostGIS Geography Type” help us here ? > > Thanks Lars > > _______________________________________________ > postgis-users mailing list > [email protected] <mailto:[email protected]> > https://lists.osgeo.org/mailman/listinfo/postgis-users > <https://lists.osgeo.org/mailman/listinfo/postgis-users>
_______________________________________________ postgis-users mailing list [email protected] https://lists.osgeo.org/mailman/listinfo/postgis-users
