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README.tree
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Venkat,
I have completed the planned Shapefile quadtree mechanism. The additions
to the traditional Shapelib are found in shptree.c (functions supporting
quad tree searching and query). There are also some new prototypes for
the tree stuff in shapefil.h ... including some prototypes for functions
you don't require and hence that I haven't implemented at this time.
I have also prepared a demonstration program using the API. That is
the ``shpdumptree'' program, with the source code in shpdumptree.c. The
shpdumptree program has two functions. One is to dump an ASCII rendering
of the internal quadtree, and the other is example use of a quad tree
searching function.
Dumping the Tree
----------------
The tree dumping is done as shown below. The "-maxdepth" commandline
switch can be used to control the maximum depth, otherwise it internally
computes a ``reasonable depth'' to use based on the number of structures
in the shapefile.
warmerda@gdal[207]% shptreedump -maxdepth 6 eg_data/polygon.shp
( SHPTreeNode
Min = (471127.19,4751545.00)
Max = (489292.31,4765610.50)
Shapes(0):
( SHPTreeNode
Min = (471127.19,4751545.00)
Max = (481118.01,4765610.50)
Shapes(0):
( SHPTreeNode
Min = (471127.19,4751545.00)
Max = (481118.01,4759281.03)
Shapes(0):
( SHPTreeNode
Min = (471127.19,4751545.00)
Max = (476622.14,4759281.03)
Shapes(0):
( SHPTreeNode
Min = (471127.19,4751545.00)
Max = (476622.14,4755799.81)
Shapes(0):
( SHPTreeNode
Min = (471127.19,4751545.00)
Max = (474149.41,4755799.81)
Shapes(6): 395 397 402 404 405 422
)
( SHPTreeNode
Min = (473599.92,4751545.00)
Max = (476622.14,4755799.81)
Shapes(10): 392 394 403 413 414 417 426 433 434 447
)
)
...
A structure like the following represents one node in the tree. In
this case it cover the region of 473599.92 < X < 476622.14,and
4751545.0 < Y < 4755799.81. There are ten shapes within this region
who's shapeids are 392, 394 ... 447. This node has no children nodes.
( SHPTreeNode
Min = (473599.92,4751545.00)
Max = (476622.14,4755799.81)
Shapes(10): 392 394 403 413 414 417 426 433 434 447
)
The hierarchy of indentation is intended to show the parent, child
relationship between nodes, with the tree being deeper the further to the
right you go.
The `-v' flag to the program can be used to expand the report to include
the full information about shapes, not just their shapeid. This can result
in a report looking more like this:
...
( SHPTreeNode
Min = (478095.78,4751545.00)
Max = (481118.01,4755799.81)
Shapes(3):
( Shape
ShapeId = 448
Min = (479988.09,4753300.00)
Max = (480705.59,4754236.50)
Vertex[0] = (480136.59,4754174.50)
Vertex[1] = (480229.97,4754182.00)
Vertex[2] = (480370.09,4754200.50)
Vertex[3] = (480695.12,4754236.50)
Vertex[4] = (480687.97,4754129.50)
Vertex[5] = (480650.47,4754075.50)
Vertex[6] = (480520.62,4753948.00)
Vertex[7] = (480490.00,4753900.00)
Vertex[8] = (480499.78,4753840.50)
Vertex[9] = (480500.97,4753820.50)
Vertex[10] = (480534.75,4753660.50)
Vertex[11] = (480560.00,4753565.00)
Vertex[12] = (480574.91,4753550.50)
...
While it is possible to part the output of the shptreedump program, and
insert it into your database, my intention was that the shptreedump program
would serve as an example of how to pre-order traversal of the quad tree,
and collect the information you will need to insert into your database.
I would then expect you to write a new program based on shptreedump that
calls a C API for your database to insert objects instead of printing them
out. Alternatively there may be an ASCII format for loading tables that
you could modify the program to output.
Searching
---------
The other thing that you can do with the shptreedump program is to
perform a search on the quadtree. For instance the following shows
searching on a small region.
% shptreedump -search 471127 4751545 476622 4759281 eg_data/polygon.shp
Shape 17: not in area of interest, but fetched.
Shape 31: not in area of interest, but fetched.
Shape 52: not in area of interest, but fetched.
Shape 76: not in area of interest, but fetched.
Shape 82: not in area of interest, but fetched.
Shape 104: not in area of interest, but fetched.
Shape 124: not in area of interest, but fetched.
Shape 134: not in area of interest, but fetched.
Shape 139: not in area of interest, but fetched.
Shape 154: not in area of interest, but fetched.
Shape 175: not in area of interest, but fetched.
Shape 177: not in area of interest, but fetched.
Shape 185: not in area of interest, but fetched.
Shape 192: not in area of interest, but fetched.
Shape 196: appears to be in area of interest.
....
I have included this capability (and the SHPTreeFindLikelyShapes() function)
so that you can see a working example of how to search this quad tree.
Note that searching is a multi-stage affair.
First a pass is made over the quadtree, collecting the shapeids of all
shapes contained in a quadtree node for which the bounding rectangle overlaps
the search rectangle. This is all accomplished by SHPTreeFindLikelyShapes()
in shptree.c.
The second phase is to fetch the actual shapes, and verify if their bounding
box falls within the area of interest. This is necessary because the shape
will tend to have a significantly smaller bounding rectangle than the tree
node in which it is found. This can result ``false positives'' on the first
phase search, as indicated by the ``not in area of interest, but fetched''
messages above. This stage is done in the SHPTreeNodeSearchAndDump()
function in shptreedump.c.
A possible third phase is to verify that the actually line segments in the
shape actually cross the area of interest. I don't both with this as it
is complicated, and assuming that the drawing engine takes care of clipping
it is quite a bit easier to let it fall through.
Building
--------
I have added a makefile.vc to the shapelib distribution. After you have
unpacked the shapefile you should have a shapelib subdirectory. If you
cd to that directory, and enter ``nmake -f makefile.vc'' in a DOS window
you should be able to build everything with VC++ (assuming it is properly
installed and in your path).
You can also create a project in VC just including the files
shpopen.c, shptree.c and shptreedump.c, building as a Win32 console
application.
For your convenience I am including prebuild .obj files, and .exe files
in the distribution.