Changes between Initial Version and Version 1 of DODSExample1

Timestamp:

Jan 27, 2009, 3:34:14 PM (15 years ago)

Author:

jmckenna

Comment:

--

DODSExample1

@@ +1 @@
+= DODSExample1 =
+
+== DODS Example ==
+
+In this example we extract point data from a "nested sequence" server and plot it in mapserver.
+
+The server in question is http://www.epic.noaa.gov:10100/dods/wod2001/natl_prof_bot.cdp. If we look at the DDS definition (from http://www.epic.noaa.gov:10100/dods/wod2001/natl_prof_bot.cdp.dds) we see:
+{{{
+Dataset {
+    Sequence {
+        Float32 lat;
+        Float32 lon;
+        Float64 time;
+        Int32 _id;
+        Sequence {
+            Float32 P_1;
+            Float32 depth;
+            Float32 T_20;
+            Float32 S0_43;
+            Float32 BO_61;
+            Float32 pH_159;
+            Float32 NO3_182;
+            Float32 PO4_186;
+            Float32 SI_188;
+            Float32 TCO2_1751;
+            Float32 PCO2_1755;
+        } profile;
+    } location;
+} natl_prof_bot;
+}}}
+A default view of this dataset through OGR would be something like this:
+{{{
+warmerda@gdal2200[20]% ogrinfo -ro -al -so 'DODS:http://www.epic.noaa.gov:10100/dods/wod2001/natl_prof_bot.cdp'
+INFO: Open of `DODS:http://www.epic.noaa.gov:10100/dods/wod2001/natl_prof_bot.cdp'
+using driver `DODS' successful.
+
+Layer name: location
+Geometry: Unknown (any)
+Feature Count: 1000
+Extent: (0.000000, 0.500000) - (0.000000, 58.080002)
+Layer SRS WKT:
+(unknown)
+lat: Real (0.0)
+lon: Real (0.0)
+time: Real (0.0)
+_id: Integer (0.0)
+profile.P_1: RealList (0.0)
+profile.depth: RealList (0.0)
+profile.T_20: RealList (0.0)
+profile.S0_43: RealList (0.0)
+profile.BO_61: RealList (0.0)
+profile.pH_159: RealList (0.0)
+profile.NO3_182: RealList (0.0)
+profile.PO4_186: RealList (0.0)
+profile.SI_188: RealList (0.0)
+profile.TCO2_1751: RealList (0.0)
+profile.PCO2_1755: RealList (0.0)
+}}}
+You will note that the default action is to convert the subsequences into "list" attributes. These are of limited use in MapServer since filters and so forth don't work on OGR list type variables. The DODS url is converted into a form usable by OGR by prepending "DODS:" to it.
+
+For plotting the points we don't need much. Just the lat, and lon. We use the DODS "projection" concept to request only the fields we are interested in. This will dramatically reduce the amount of data that the DODS server needs to return to mapserver. In this case we might use the following OGR datasetname:
+{{{
+ogrinfo -ro -al 'DODS:http://www.epic.noaa.gov:10100/dods/wod2001/natl_prof_bot.cdp?lat,lon'
+INFO: Open of `DODS:http://www.epic.noaa.gov:10100/dods/wod2001/natl_prof_bot.cd
+p?lat,lon'
+using driver `DODS' successful.
+
+Layer name: location
+Geometry: Unknown (any)
+Feature Count: 1000
+Extent: (0.000000, 0.500000) - (0.000000, 58.080002)
+Layer SRS WKT:
+(unknown)
+lat: Real (0.0)
+lon: Real (0.0)
+OGRFeature(location):0
+  lat (Real) = 0.5
+  lon (Real) = 0
+  POINT (0.00000000 0.50000000)
+...
+}}}
+To minimally plot the data, we might use a .map file like:
+{{{
+MAP
+NAME  "test"
+SIZE  400 400
+EXTENT  -30 0 30 60
+
+PROJECTION
+"proj=latlong"
+END
+
+SYMBOL
+  NAME "cross"
+  TYPE vector
+  POINTS
+    2 0
+    2 4
+    -99 -99
+    0 2
+    4 2
+  END 
+END
+
+LAYER
+  NAME  "test"
+  TYPE POINT
+  CONNECTIONTYPE OGR
+  CONNECTION 'DODS:http://www.epic.noaa.gov:10100/dods/wod2001/natl_prof_bot.cdp?lat,lon'
+  DATA 'location'
+  STATUS  default
+  CLASS
+    COLOR 255 255 0
+    SYMBOL 'cross'
+    SIZE 8
+  END
+END
+
+END
+}}}
+As per normal OGR usage, the datset name is the CONNECTION string, and the layer name is in the DATA statement. In this case symbol yellow cross symbols are used to plot the points.