People:
There is a potential problem with the WITHIN operator in the Mapinfo SELECT
statement. The problem is that the result set produced by a
SELECT...WITHIN can incorrectly contain DUPLICATE RECORDS.
In my case, this behaviour initially lead to significant overcounting errors
in a survey analysis project, and took some time to isolate and find a
work-around.
The trouble can arise when you try to select and count POINT objects from
one table using a set of adjacent REGION objects in a second table. For
example, if Client is a POINT table and SurveyArea is a REGION table
containing a group of Census Tracts that +ACI-tile+ACI- the survey area then the
statement
Select Client.+ACo-
From Client, SurveyArea
Where SurveyArea.Obj contains Client.Obj
will sometimes include duplicate copies of Clients.
What happens is that any Client point that falls right on the boundary
between two census tracts IS SELECTED IN BOTH REGIONS BY THE WITHIN
OPERATOR, and is thus double-counted.
In my case the work-around was to explicitly pre-select and combine all the
CensusTract objects to create a single SurveyArea region, thus eliminating
any double-counting caused by the internal boundaries.
The same phenomenon occurs in both MapInfo Pro 5.0 and MapInfo Pro 5.5.
David M Haycraft Phone/Fax: 61 +- 2 +- 6231 8104
Information Analysis Assocs P/L Mobile: 0412 001 134
ACN 085 516 105 Email:
d.haycraft+AEA-acslink.aone.net.au
1 Cumming Place, Wanniassa Web:
www.acslink.aone.net.au/actaa/iaa.htm
A.C.T. 2903, Australia A MapInfo Partner
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