I've not been able to resolve this issue yet, but this post is just an
update as to different things I've tried that haven't worked ;)
Firstly, just to confirm my Mapnik and OGR installations are working
correctly, I created the most simple of CSV files, containing a single
Polygon:
ID,WKT
1,"POLYGON((0 52, 4 52, 4 60, 0 60, 0 52))"
Then I created an OGR VRT file to point at this CSV file, as follows:
<OGRVRTDataSource>
<OGRVRTLayer name="test">
<SrcDataSource>test.csv</SrcDataSource>
<LayerSRS>WGS84</LayerSRS>
<GeometryType>wkbPolygon</GeometryType>
<GeometryField encoding="WKT" field="WKT"/>
</OGRVRTLayer>
</OGRVRTDataSource>
Tested the CSV file using OGRINFO:
ogrinfo -al csv.vrt
INFO: Open of `csv.vrt'
using driver `VRT' successful.
Layer name: test
Geometry: Polygon
Feature Count: 1
Extent: (0.000000, 52.000000) - (4.000000, 60.000000)
Layer SRS WKT:
GEOGCS["WGS 84",
DATUM["WGS_1984",
SPHEROID["WGS 84",6378137,298.257223563,
AUTHORITY["EPSG","7030"]],
TOWGS84[0,0,0,0,0,0,0],
AUTHORITY["EPSG","6326"]],
PRIMEM["Greenwich",0,
AUTHORITY["EPSG","8901"]],
UNIT["degree",0.0174532925199433,
AUTHORITY["EPSG","9108"]],
AUTHORITY["EPSG","4326"]]
ID: String (0.0)
WKT: String (0.0)
OGRFeature(test):1
ID (String) = 1
WKT (String) = POLYGON((0 52, 4 52, 4 60, 0 60, 0 52))
POLYGON ((0 52,4 52,4 60,0 60,0 52))
And successfully plotted the polygon feature from this VRT using Mapnik:
import sys
import mapnik
m = mapnik.Map(600,800,"+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0
+lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0 +units=m +nadgrids=@null +no_defs")
m.background = mapnik.Color('blue')
# Create the layer from VRT
test_lyr = mapnik.Layer('Test layer')
test_lyr.srs = "+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs"
test_lyr.datasource = mapnik.Ogr(file="csv.vrt", layer="test")
# Define a simple fill
test_style = mapnik.Style()
test_rule = mapnik.Rule()
test_rule.symbols.append(mapnik.PolygonSymbolizer(mapnik.Color('red')))
test_style.rules.append(test_rule)
# Apply the style
m.append_style('test-style', test_style)
test_lyr.styles.append('test-style')
m.layers.append(test_lyr)
# Set the initial extent of the map
m.zoom_to_box(mapnik.Envelope(-1252344,6261721,626172,8140237))
# Render and save the map image
im = mapnik.Image(m.width,m.height)
mapnik.render(m, im)
im.save('demo.png', 'png') # true-colour RGBA
Having established that's all working, I now tried exactly the same polygon,
but selected via a SQL Server query. Note that I'm doing this all in a
simple inline query so there's no dependency on any tables existing - it
just requires a successful connection to SQL Server. I made no other changes
to the VRT file:
<OGRVRTDataSource>
<OGRVRTLayer name="test">
<SrcDataSource>MSSQL:server=zangief\DENALICTP3;database=TempDB;</SrcDataSour
ce>
<SrcSQL>SELECT geography::STPolyFromText('POLYGON((0 52, 4 52, 4 60, 0
60, 0 52))', 4326).STAsText() AS WKT</SrcSQL>
<LayerSRS>EPSG:4326</LayerSRS>
<GeometryType>wkbPolygon</GeometryType>
<GeometryField encoding="WKT" field="WKT"/>
</OGRVRTLayer>
</OGRVRTDataSource>
OGRINFO reports *exactly the same* as it did using the CSV source:
ogrinfo -al mssql.vrt
INFO: Open of `mssql.vrt'
using driver `VRT' successful.
Layer name: test
Geometry: Polygon
Feature Count: 1
Extent: (0.000000, 52.000000) - (4.000000, 60.000000)
Layer SRS WKT:
GEOGCS["WGS 84",
DATUM["WGS_1984",
SPHEROID["WGS 84",6378137,298.257223563,
AUTHORITY["EPSG","7030"]],
AUTHORITY["EPSG","6326"]],
PRIMEM["Greenwich",0,
AUTHORITY["EPSG","8901"]],
UNIT["degree",0.0174532925199433,
AUTHORITY["EPSG","9122"]],
AUTHORITY["EPSG","4326"]]
ID: Integer (10.0)
WKT: String (0.0)
OGRFeature(test):0
ID (Integer) = 1
WKT (String) = POLYGON ((0 52, 4 52, 4 60, 0 60, 0 52))
POLYGON ((0 52,4 52,4 60,0 60,0 52))
However, Mapnik fails with the error described previously - "Failed to open
datasource".
I ought to mention that both OGR2OGR and QGIS do load this VRT without any
problems, so I'm confident that the connection string and query are correct.
I also tried changing the VRT to connect to SQL Server via ODBC:
<OGRVRTDataSource>
<OGRVRTLayer name="test">
<SrcDataSource>ODBC:Driver={SQL
Server};Server=zangief\DENALICTP3;Database=tempdb;UID=;PWD=</SrcDataSource>
<SrcSQL>SELECT 1 AS ID, geography::STPolyFromText('POLYGON((0 52, 4 52,
4 60, 0 60, 0 52))', 4326).STAsText() AS WKT</SrcSQL>
<LayerSRS>EPSG:4326</LayerSRS>
<GeometryType>wkbPolygon</GeometryType>
<GeometryField encoding="WKT" field="WKT"/>
</OGRVRTLayer>
</OGRVRTDataSource>
Exactly the same result as previously - OGR2OGR and QGIS will load the
features from this VRT without any problem, but Mapnik won't. So I'm
inclined to agree that this is a problem somewhere in the Mapnik OGR plugin,
which for some reason is affecting a SQL Server connection but perhaps not
other OGR sources (including CSV). I'm not sure what the significance of
opening a datasource in SHARED mode is, so I don't know if that's the likely
cause or not.
I appreciate that there probably aren't that many Mapnik users who have
tried to use SQL Server (in fact, from looking around the internet, it
appears that I might be the only one!) or, in fact, whether anybody has been
successful in doing so yet, but native SQL Server support would be a massive
bonus for people trying to use Mapnik on windows, where SQL Server Express
is generally a more prevalent DB than PostGIS, say.
Is there anything else I can do to help identify/fix the root cause of this
problem?
Thanks,
a.
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