Joining tables to produce thematic maps
collection of data for a GIS, there may be many different
Often there will be a need to join these separate tables
of information to produce a query. In this case there will
normally be a one-to-one correspondence between records in
the two tables, although they not be in the same order.
As an example of this joining procedure, we will use the
national dataset of malarial statistics supplied by IMPE.
The two tables selected are ‘Population_Malaria_Risk’ and
"Spray_Insecticide_Impregnation’. These tables are part of
a set of tables that have been created for the Vietnam-EC
Malaria Control Project.
Both tables pertain to the seven provinces that constitute
the project’s area of interest. The data has been compiled
from aggregated District reports for 1997.
concerns data estimating the population at risk of
The other records the distibution of insecticide for
Our query will map the result of the join, showing the
ratio of population protected to that of population at
The SQL is shown at right.
To carry out this join we create a very simple SQL enquiry
using Query>SQL Select. When we select the two tables
‘Population_Malaria_Risk’ and "Spray_Insecticide_Impregnation’,
MapInfo automatically inserts the following join clause in
the Where Condition:
When doing Table>Update Column, Thematic Mapping, and SQL
Select with only two tables, MapInfo will automatically
set the join condition.for you, based on matching field
type and content.
statement can be saved for use in a later session. Click
onto the Save Template button in the SQL dialog and the
Save Dialog to Query File dialog (as displayed to left)
Choose path and name of query, as required. Any temporary
tables created are otherwise lost when MapInfo is closed.
If the query results are only required in this session, or
if the query can be rerun easily, use Map>Layer Control
(Ctrl-L) and Add the table just created. Objects resulting
from the query can be distinguished from the original
table from which they are derived by changing their
display style using the Display button on the layer
control dialog, or the original table can be made
If a permanent table is required, it can be saved by using
File>Save Copy As.
Whichever method is used, the next step is to use
Map>Create Thematic Map. Choose Ranges as the method of
In Step 2 of
Create Thematic Map, choose the bottom listing
(Expression…) in the pull down menu for Fields.
In the Expression dialog, select the Columns
‘Total_Pop_protected’ and ‘Pop_at_Malaria_Risk’. Make sure
the Divisible oprator,’/’, is located between the two
column names (see right).
Thematic Map is displayed to right.
Note that there is inconsistency to some of the data
figures. The ratio of protected population should not
exceed the total population at risk. Yet in two of the
provinces (Kon Tum and Dac Lac) this ratio has been
This anomaly highlights the intrinsic relationship that
exists between the quality of data input and the accuracy
of the final output. In this instance, there has either
been considerable over-reporting or a disproportionate
aggregation of data at provincial level.
This issue of data management is a core concern to GIS
output. By the use of thematic shading it is possible to
verify data integrity at a glance.
Different layers of thematic styles can be overlaid over
Bar Graphs, Pie Charts and Dot Density thematics can all
be applied, depending on the choice of data.
For this example, we will use the Graduated symbols to
display the column of ‘Deaths_1000_Population’.
default colour is red, and the previous colour range was
also red, it is better to select another colour. This is
achieved by customizing the Settings and nominating blue
instead. The Legend title can be altered similiarly.
Care must be taken not to overcrowd the map with too much
Cartographic issues regarding scale, generalisation and
projection are all considerations in designing a good
also be paid to consistently spaced ranges whenever
Ranging a thematic map. This can be achieved by
Customising Ranges in Step 2 of Creating a Thematic Map.
Select the Custom Method in the top pull down menu.
If you open
the Layer Control dialog, you will see that the thematic
layers are indented above the parent table from which the
thematics are derived. They are not editable. They can be
re-ordered or turned off (see below).