RE: MI Buffers with problems...

[Warren Vick, Europa Technologies Ltd.]

Hello Ian,

If I understand your message correctly, the bug you describe is not related
to the accuracy issue of passing objects through the cosmetic layer but one
regarding MapInfo's inability to create large spherical buffers correctly.
This is has been a long standing bug in Pro - since I started to use v3.0
anyway. The problem is demonstrated by creating large buffers (typically
several hundred miles) around objects. Pro converts a single lat/lon (at the
centroid I'm told) and turns the buffer distance into a unit measurement of
latitude and longitude. The problem here is that a degree of longitude
changes width depending on latitude. The single sample point means that
buffers always appear elliptical (in the normal rectangular world
projections). I have also observed that the "spread" which should be seen as
a buffer approaches the poles does not occur. Changing projection does not
seem to have any effect as the buffer functions do not take coordinate
system into account (unless deciding between spherical and Cartesian
schemes). The errors induced can be quite staggering (I've seen ~20%),
especially if you create a large buffer around a latitude rich region (like
the country Chile).

If this is the problem you are experiencing, then as you say, it has been
reported and it has come up on the list a couple of times. The separation of
spherical and Cartesian buffers highlights the problem more-so since Pro's
implementation is not a spherical calculation at all - more a sort of
adaptive linear system. Personally, I think it's the most serious bug in the
product. I have not found any workaround apart from having to write my own
spherical buffer code. Extreme caution is needed in this area for safety or
commercially critical applications.

BTW, please take it easy on those attachments when posting to the list!

Regards,
Warren Vick
Europa Technologies Ltd, U.K.
http://www.europa-tech.com

> -----Original Message-----
> From: [EMAIL PROTECTED]
> [mailto:[EMAIL PROTECTED]]On Behalf Of Ian Erickson
> Sent: Friday, December 17, 1999 11:42 PM
> To: [EMAIL PROTECTED]
> Subject: MI Buffers with problems...
>
>
> Here's a challenge for those of you who are willing to accept:
>
> I know there has been extensive documentation of buffers created in the
> Cosmetic Layer being elliptical.  I understand that MapInfo is
> aware of the
> problem and is working to fix it.  The proposed work-around is to
> create the
> buffers in a MapInfo table that has a projection.  So if I was
> 'Joe User', I
> might find a point layer make the layer editable, select
> Objects->Buffer...
> and then create the buffers in the same layer as the points.  Once the
> buffers are created I would save the selected buffers as a new table, and
> all is good right?  Wrong!
>
> When 'Joe User' does this, (ie the Objects->Buffer... menu
> command) MapInfo
> executes the command:
>
>       Create Object As Buffer
>               From Selection
>               Into Table {sourcetable}
>               Width 200
>               Units "mi"
>               Resolution 50
>               Group By RowID
>
> Guess what, if your layer is somewhat dispersed over the earth,
> the objects
> on the edges are more and more deformed.  Meaning: they become elliptical.
> I've attached an .MBX, source code and the MapInfo table used to recreate
> the problem.  All this program does is compute the distance from
> the center
> of the circle, to the nodes on the edge of the circle.  In an ideal world,
> you would expect to see the 200 miles from the center of the
> circle to each
> of the points comprising the buffer.  For those of you out there who are
> experts in the field of projections, datums and spherical
> coordinates, help
> me out here.  I ran the same program on a version of 4.5 and the
> problem did
> not occur, however, 5.5 really messes things up.  I've got a
> work-around for
> myself, which is slower and more time consuming, but I suggest
> that this is
> a bug NOT a feature.
>
> Ian Erickson
> GIS Analyst
>  <<AllPoints.DAT>>  <<AllPoints.ID>>  <<AllPoints.IND>>
> <<AllPoints.MAP>>
> <<AllPoints.TAB>>  <<test.mb>>  <<test.MBX>>
>

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