Mpaka,
There are several possible reasons and solutions for your classification
generating very large vector files:
- The raster to vector conversion outlines *each* different cell exactly.
If you have a classification with many different types of classes, that
vary pixel by pixel, the output vector can be huge.
- While this is technically correct, most GIS systems choke on such large
complex vector polygon files.
- So the problem is that the raster to vector *is* giving you the "correct"
result (and hense very large files) but this is not ideal when processing
the vector polygones, as most of the time you want a simpler representation.
- For this reason, classifications are often "smoothed" to clump
classes. This dramatically reduces the numbers of polygons
generated - but the results of course are not a true literal exact
match to the original classification.
- There are a number of "smoothing" filters you can use. Because the
values are classes, you do not want a normal smoothing filter. Instead,
you will want to use something like a median filter, or better yet,
the Majority filter or the Adaptive Median filter.
- You can find these filters in the "filters_ranking" filters
directory. By default most of the filters use a 5x5 filter; you can
change this to larger or smaller filter sizes as required. The larger
the filter, the simpler different classes will be (less individual pixels
of a different class). The Majority filter might work well for you at 7x7.
- Add the filter to an algorithm, then feed that algorithm through the
raster to vector instead of using the original classified imaged.
This will make a dramatic reduction on the size and complexity of the
output vector file.
- If you also use the "smooth" option in the raster to vector converter,
it will do some small smoothing of the vector polygon, rather than
trying to follow the outline of each pixel. This can also reduce the
size of the file.
As an aside, most of the vector formats (like dxf) are ascii format files, so
they can be quite large.
Regards
Stuart
> Dear List,
>
> We converted our raster landcover classes to vector (erv) and subsequently
> to dxf but the dxf files generated are so large (greater than the size of
> the raster data) that it makes nonsense of the entire process.
>
> Please are we doing something wrong or is there a bug in the process?
>
> Thanks for anticipated response.
>
> Mpaka
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