MapInfo Pro provides a number of powerful aggregating functions that allow you to derive new information. These features are available in Update Column when you specify two tables in the Update Column dialog box. The aggregate expressions include Average, Count, Minimum, Maximum, Sum, Weighted Average, Proportion Sum, Proportion Average, and Proportion Weighted Average. Each is defined in the next table.
| Aggregate Expressions | Description |
|---|---|
|
Average |
Calculates the average of the values for all records in a group. |
|
Count |
Counts the number of records in a group. |
|
Minimum |
Finds the lowest value for all records in a group. |
|
Maximum |
Finds the highest value for all records in a group. |
|
Sum |
Calculates the sum of the values for all records in a group. |
|
Weighted Average |
Gives more weight to one value over another when averaging. |
|
Proportion Sum |
A sum calculation that is adjusted based on how much of one object is within another object. |
|
Proportion Average |
An average calculation that is adjusted based on how much of one object is within another object. |
|
Proportion Weighted Average |
A weighted average calculation that is adjusted based on how much of one object is within another object. |
Note that average, count, min, max, sum, and weighted average operate on data values only. The proportion functions take geographic relationships into account.
For faster performance for proportional calculations, use the Proportional Overlap Tool; see Using the Tools in the Tools Manager.
Example Using Proportion Sum and Proportion Average
To illustrate the proportion sum and proportion average functions, consider this example.
You want to determine what proportion of the population in your county is covered by a flood zone. You have two tables: a Towns table that contains town boundaries and population statistics, and a Flood table that contains a region object (circle in illustration) that represents the area of past flooding.
The flood zone covers three towns. As shown in the following illustration, MapInfo Pro calculates the proportion of the population in those three towns that is within the flood zone and reports the sum in a temporary column in the Flood table. Similarly, MapInfo Pro calculates the proportion average of property values for the same areas.
Proportion sum is best used for raw data (population, housing units, etc.) while proportion average is used for derived data (median income, average housing costs, etc.).
Example Using Proportion Weighted Average
Proportion Weighted Average is similar to proportion average except that you add a weighting factor to the average calculation. That weighting factor is another data column in your table.
For example, you want to find the best location in the area to build a new upscale shopping mall. You believe the mall will draw people from a 20-mile radius. You are considering five potential sites and you want to place the mall where you can find the optimum combination of population and income.
To do this, create a temporary column in your Mall table with data from your Towns table that contains the proportion of each town's population affected by the potential mall. Use the median income for each town as the weighing factor. In this example MapInfo Pro determines the join automatically to be where the town boundaries intersect the 20-mile buffers around the potential sites.
The best location of the mall will be at the site that returns the highest value of the temporary column, as shown in the Browser in the next figure.