Joining tables to produce thematic maps

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Joining tables to produce thematic maps

In the collection of data for a GIS, there may be many different tables created.

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.

One table concerns data estimating the population at risk of malarial infection.

The other records the distibution of insecticide for vector control.

Our query will map the result of the join, showing the ratio of population protected to that of population at risk.

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:

Population_Malaria_Risk.Code = Spray_Insecticide_Impregnation.Code

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.

The SQL 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) will appear.

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 non-visible.

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 thematic display

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).

The resulting 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 exceeded.

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 each other.

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’.

Because the 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 information.

Cartographic issues regarding scale, generalisation and projection are all considerations in designing a good thematic map.

Attention must 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).

 
   
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