Changes between Version 1 and Version 2 of UsersWikiBufferCoast

Timestamp:

11/27/12 17:36:22 (12 years ago)

Author:

spluque

Comment:

—

UsersWikiBufferCoast

@@ -1 @@
-In morphology, the dilation operation followed erosion is an extremely useful technique that allows for the simplification of complex shapes.  For example, the technique can be used to produce a simplified coastline, where bays and fjords are ignored, and that can later be used as a mask for further operations.
+In morphology, the dilation operation followed by erosion is an extremely useful technique that allows for the simplification of complex shapes.  For example, the technique can be used to [http://blog.opengeo.org/2010/11/22/removing-complexities produce a simplified coastline], where bays and fjords are ignored, and that can later be used as a mask for further operations. Below is an illustration of the process of this technique for simplifying the convoluted coastline of Newfoundland, Canada, using GSHHS.
+
+At first glance, it would seem that simply using one of the lower resolution coastline datasets from GSHHS would achieve the same result, but they were generated using a different procedure, which is not equivalent.
+
+= Import GSHHS polygons into PostGIS =
+
+[ftp://ftp.soest.hawaii.edu/pwessel/gshhs/GSHHS_shp_2.2.0.zip GSHHS shapefiles] provided by the maintainers can be imported into a PostGIS database (named gshhs here) using:
+
+{{{
+shp2pgsql -s 4326 -I -g geom GSHHS_f_L1.shp gshhs_f_l1 | psql -d gshhs
+}}}
+
+= Subset coastlines =
+
+Before performing any dilation/erosion, it is helpful to subset the coastline polygons for efficiency purposes.  The {{{gid}}} field in the table holding the GSHHS data identifies a single polygon.  The island of Newfoundland has {{{gid=23}}}:
+
+{{{
+CREATE OR REPLACE VIEW newfoundland_subset_f
+SELECT gshhs_f_l1.gid, gshhs_f_l1.geom
+   FROM gshhs_f_l1
+  WHERE gshhs_f_l1.gid = 23;
+}}}
+
+= Dilation and erosion =
+
+PostGIS can perform these two operations via {{{ST_Buffer}}}.  For the purposes outlined here, the process involves buffering positively (dilating) the full resolution coastline by a given amount {{{k}}}, and then buffering the result negatively (eroding) by the same amount.  The following shell script applies the technique using different values for {{{k}}}.  It uses the utility {{{ogr2ogr}}} to export the PostGIS objects to an ASCII table in GMT (Generic Mapping Tools) format:
+
+{{{
+PREFIX="newfoundland"
+COAST="newfoundland_subset_f.gmt"
+BUFOUT="newfoundland_subset_f_bufout"
+BUFIN="newfoundland_subset_f_bufin"
+
+ogr2ogr -f GMT ${COAST} \
+    PG:"host=localhost user=username password=password dbname=gshhs" \
+    ${COAST%%.gmt}
+for buf in ${BUFFERS[@]}; do
+    cat <<EOF > ${GMT_TMPDIR}/buffers.sql
+    CREATE OR REPLACE VIEW ${COAST%%.gmt}_bufout AS
+    SELECT gid, ST_Buffer(geom, ${buf}) FROM ${COAST%%.gmt};
+    CREATE OR REPLACE VIEW ${COAST%%.gmt}_bufin AS
+    SELECT gid, ST_Buffer(st_buffer, -${buf}) FROM ${COAST%%.gmt}_bufout;
+EOF
+    psql -f ${GMT_TMPDIR}/buffers.sql gshhs
+    ogr2ogr -f GMT ${COAST%%.gmt}_bufout${buf}.gmt \
+        PG:"host=localhost user=username password=password dbname=gshhs" \
+        ${COAST%%.gmt}_bufout
+    ogr2ogr -f GMT ${COAST%%.gmt}_bufin${buf}.gmt \
+        PG:"host=localhost user=username password=password dbname=gshhs" \
+        ${COAST%%.gmt}_bufin
+done
+}}}
+
+The animated {{{GIF}}} image below illustrates the results.