At 09:37 PM 3/24/2011 +1100, Michael Bedward wrote:
Hi John,
> However, I don't know how to remove the coincident vertices. I am working
> with a FeatureCollection from a shapefile, and the coincident vertices are
> in polygons in separate features. Also, I think what I really need to do is
> remove lines segments (2 or more consecutive vertices) that are covered by
> line segments from a different feature.
>
> Is there a way to do that short of manually picking apart the
geometries and
> doing some sort of spatial search?
If the vertices are coincident or nearly so, in terms of proximity to
other vertices or they fall on line segments belonging to other
polygons, then you should be able to remove then with
TopologyPreservingSimplifier. I tried to show that with the example I
posted.
This is the second issue related to simplifying with multiple geometries.
It is related to, but distinct from the prior thread.
Stated in non-GIS terms:
I have two kinds of United States political boundaries:
States: relatively large areas. Described by multiple polygons
Counties: smaller areas. Each county is contained with a single state,
and every state is completely filled with counties.
On a map, I wish to show state boundaries in one color, and county
boundaries in another color (and lower Z-order so state boundaries hide
county boundaries where they are coincident).
The data I currently have (US National Atlast shape files) meet these criteria.
GOAL:, I need to simplify the features - I don't need the full precision of
the data, and I need the number of lines to be reduced for storage and
processing reasons.
THE PROBLEM:
If simplify counties independently from states,county boundary lines which
were included (in a colinear fashion) in state boundary (and hence hidden
by the state boundary layer) are now no longer colinear. Hence when then
two are displayed together, they look bad. As per the previous thread,
adjacent county boundaries also become non-colinear unless county
geometries are combined.
I don't know how to achieve this using the tools.
I have illustrated thisi in character graphics below (using Fixedsys font):
BEFORE: (outer boundary is state boundary)
...-------------------------------+
| |
| County A |
| \
| \
| __+
| |
| +-+ |
County C +---------/ |--------+
| |
| County B /
| /
| --+
| /
| /
...-----------------------------+
DESIRED AFTER:
...-------------------------------+
| |
| County A |
| |
| |
| |
| |
| |
County C +---------------------+
| |
| County B /
| |
| |
| /
| /
...-----------------------------+
RESULT OF SIMPLIFYING STATES AND COUNTIES INDEPENDENTLY (to illustrate that
different lines, colinear, become not colinear when simplified separately)
...-------------------------------+
| |
| County A |
| \
| |\
| |/
| |
| /\ |
County C +---------------------+
| |
| County B /|
| \|
| +
| /
| /
...-----------------------------+
Thanks
John
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