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| Subject: | [gislist] MapInfo Enquiry - reducing the number of nodes - what
approach is best? |
| Date: |
09/25/2003 11:40:01 AM |
| From: |
Patricia Connor Reid |
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Can anyone provide some insight on how best to proceed with the task
of reducing the number of nodes on a very detailed file?
I am currently working with a series of detailed, high resolution
boundary files from DMTI for Southwestern Ontario that were developed
by the authors for mapping (and analysis) as low as the street scale.
Since we are using them for a much smaller scale (larger area) they
have far more nodes and decimal places (6) than are appropriate for
immediate needs.
We need to reduce the number of nodes both for design purposes
(inlets become congested into 'globs' due to the highly detailed
boundary file being used to depict a large area), and for the
purposes of the map importing in as small a file size as possible for
subsequent SVG programming (for a web browser-based mapping program).
I am in the steep section of the learning curve with respect to such
tasks in MapInfo and seek your input on three approaches to reducing
the number of nodes in the file size.
1) Thinning the nodes.
I need to reduce the number of nodes in the hydrography layer in many
areas along coastlines in order to generalize the linework so that at
smaller scales, the inlets have some white space in them instead of
being black globs due to the overly detailed linework for that scale.
I have tried using the filter to thin the nodes (Object - Snap/Thin
with Enable Node Thining turned on. While the action takes place,
the boundary file polygon disappears altogether and I get an error
message as follows:
"All or part of Objects invalid. Results may be incorrect. Warning:
Operation did not produce a result. No errors detected."
Any ideas what is going wrong? Could it be that the polgons in this
boundary file are not in fact closed? If so, how do I check for that?
2) Manual editing of nodes through Tracing Existing Objects OR Just
Manually Editing the Nodes
Would it be more useful to click on 'Snap Node' and begin tracing
from node to node selectively to accomplish what I have in mind?
That would allow me to have more control over the generalization. Is
this quicker than just manually removing the nodes, one by one?
Since it is a rather large area that I'm working with, the method
that is most time efficient will have major ramifications. This will
not be a half hour job.
3) Reducing the number of decimal places in the boundary file.
Is it possible to also reduce the number of decimal places that
describes the node locations (currently to six decimal places, when
we perhaps only need 3 for our purposes). I'm querying this approach
as I'm thinking that then perhaps I could use the 'Clean Objects'
filter to remove overlaps and remove gaps...but again perhaps I'd run
into the problem of the filter working properly on the boundary file
as in point #1.
The reason, by the way, that reducing the number of decimal places is
relevant in this particular application is that I understand from the
programmer that is working on this project as well, that when the
boundary files move into the SVG world, EVERY character in the SVG
file enlarges the file. If we can drop off 3 characters from each of
the georeferencing entries, this cuts the file size to significantly
smaller size.
Thank you for your thoughts on this.
Patricia Connor
--
"I am not difficult to please: I am always satisfied with the best."
Oscar Wilde
Mrs. Patricia Connor Reid,
Director of Cartographic Services,
Geography Department,
Room 1302, Social Science Centre,
The University of Western Ontario
London, ON Canada N6A 5C2
Phone: 519-661-3425: email: connor@uwo.ca: fax: 519-661-3750:
webpage: http://geography.ssc.uwo.ca/staff/connor.htm
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