Download Spatial programming is just combining any programming language

Open Source Geospatial
Spatial Programming
Barry Rowlingson
School of Health and Medicine,
Lancaster University
1
Spatial programming is just combining any
programming language with spatial data.
Now because spatial data can be so complicated,
you may find someone has already done some of
the work for you.
Laziness
C
C++
Python
GDAL
DATA
OGR
R
Fortran
2
Laziness is one of the three virtues of programmers. I
can't be bothered to tell you the other two, go look
them up.
Laziness means we never do anything more times
than we have to. We don't reinvent that wheel. So If
we want to get to our precious geodata, we use the
existing libraries – most likely GDAL for rasters and
OGR for vectors.
”bindings” to different languages exist – for gdal and
ogr the most common ones are C, C++, and
Python, and there's also a way to use them from R.
Sadly Fortran programmers may be a bit stuck. There
might be a way in via the C library, but I'm not sure
anyone has ever attempted it. Good luck.
So what does it look like to use these libraries?
OGR in Python
>>> import ogr
>>> europe = ogr.Open(”europe.shp”)
>>> europe
<ogr.DataSource instance at 0xb59f2c0c>
>>> layer = europe.GetLayer(0)
>>> layer
<ogr.Layer instance at 0xb59f2c8c>
>>> f = layer.GetFeature(3)
>>> [f.GetFieldDefnRef(i).GetName() for i in range(f.GetFieldCount())]
['CNTRY_NAME', 'COUNT', 'FIRST_FIPS', 'FIRST_REGI', 'FIRST_CONT', 'SUM_POP_AD', 'SUM_SQKM_A', 'SUM_SQMI_A']
>>> f.GetFieldAsInteger("COUNT")
11
>>> f.GetFieldAsString("CNTRY_NAME")
'Belgium'
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Here's how you do it in Python.
Python is an easy to learn general purpose
programming language, with lots of extra modules.
Here I'm using the ogr module. I import it, and then I
can use it. I can open a shapefile just like that. Or
any other data type that OGR supports. The details
are hidden from me.
Once I have the shapefile read in, I can query it.
Technically now its an OGR Data Source, and it
has Layers. So I get the first layer (which python
calls layer zero). Now I can query that layer.
Layers have Features, so I get the fourth feature
(they start with zero) and now I have a feature
object. I can query that. Here's the names of its
attributes and the value of two of them. We now
know that the fourth feature in Europe.shp is
Belgium.
OGR in Python
>>> geom = f.GetGeometryRef()
>>> geom.GetGeometryType() == ogr.wkbPolygon
True
>>> geom.GetArea()
3.89416677221379
>>> geom.GetEnvelope()
(2.5416665077209473, 6.3982038497924805, 49.504165649414062, 51.503608703613281)
>>> geom.ExportToGML()
'<gml:Polygon><gml:outerBoundaryIs><gml:LinearRing>
<gml:coordinates>3.182874202728271,50.757049560546875 3.164721965789795,50.780548095703125 3.162221908569336,50.783607482910156
3.154444217681885,50.788330078125
3.149722099304199,50.789993286132812 3.143332958221436,50.79083251953125 .....
4
As well as the attributes you can also query the
geometry of a feature. Here I've confirmed that
what I've got are polygons, I've got the area of
Belgium (in square degrees, so not a very useful
measure), and the coordinates of the bounding
box. Then I've converted it to GML.
So I can get in a few lines of Python all my geoinformation. And once it's in my programming
language I can do anything with it. And then I can
write it back out into maps or tables of summaries
or stick it in a database or email it to my friends or
whatever.
OGR in C++
OGRRegisterAll();
OGRDataSource *poDS;
poDS = OGRSFDriverRegistrar::Open( "point.shp", FALSE );
if( poDS == NULL )
{
printf( "Open failed.\n" );
exit( 1 );
}
OGRLayer *poLayer;
poLayer = poDS­>GetLayerByName( "point" );
OGRFeature *poFeature;
poLayer­>ResetReading();
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Here's how to do a similar thing in C++. The concepts
are the same – data source, layer, feature – but the
syntax is different because this is C++ and not
Python. If you understand one, you can understand
the other. Laziness.
OGR in C++
poFeature = poLayer­>GetNextFeature()
OGRFeatureDefn *poFDefn = poLayer­>GetLayerDefn();
int iField;
for( iField = 0; iField < poFDefn­>GetFieldCount(); iField++ )
{
OGRFieldDefn *poFieldDefn = poFDefn­>GetFieldDefn( iField );
if( poFieldDefn­>GetType() == OFTInteger )
printf( "%d,", poFeature­>GetFieldAsInteger( iField ) );
else if( poFieldDefn­>GetType() == OFTReal )
printf( "%.3f,", poFeature­>GetFieldAsDouble(iField) );
else if( poFieldDefn­>GetType() == OFTString )
printf( "%s,", poFeature­>GetFieldAsString(iField) );
else
printf( "%s,", poFeature­>GetFieldAsString(iField) );
}
OGRGeometry *poGeometry;
OGRPoint *poPoint = (OGRPoint *) poGeometry;
printf( "%.3f,%3.f\n", poPoint­>getX(), poPoint­>getY() );
6
And here's how you get the feature attributes and the
geometry. This example actually loops over all
attributes of the feature and prints the data out.
So how do you figure out how to do all this stuff?
Well you read the documentation, learn about the
architecture, and consult the API.
API
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The API is the public face of a library. It tells you what
you can do with it. This page from the OGR
documentation explains the fundamentals – data
sources, layers, features, attributes and so on.
OGR reference api
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From there you can click through to get
documentation for specific functions in the library –
here is the help for the Open function that opens a
data source. This example is for C++.
There's another good source of help, and that is
other people's source.
Use The Source, Luke!
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Here's a chunk of the source code from the OGR
library. It's open source, so you can read and learn.
It's also very neatly laid out with hyperlinks between
functions and clicks to get you to the
documentation. You can very rapidly jump around
in the code and as long as you dont get lost you'll
learn stuff pretty quickly and soon be writing your
own programs.
Virgilio is now going to show you some applications
of spatial programming using the R programming
language.