Rummaging in the Toolbox:
Displaying Data in Grids Using the Grid Maker tool
| ||I know I am often challenged to create maps in which I want to easily convey a message but the amount of data is overwhelming. Take for example the following two maps. The first map simply displays a large number of points. The second map uses the same data but with the points aggregated to 5 mile grid cells. |
If the goal of the map is to convey the areas of a higher concentration of points versus the areas with a lower concentration of points, which map of the following two maps would you prefer?
This article will show you how to create a grid of squares in which you can aggregate and display point data.
To create the grid map, I use MapInfo Professional's Grid Maker tool. To get started, click on Tools > Tool Manager and load the Grid Maker tool.
When you run the Grid Maker tool the Tools tool bar will appear. It will include a button for creating a grid. See the screen shot below.
Using the tool, click and drag to create a box around your data points. The Grid Maker dialog box will display:
Now, the hard part: What to enter into the dialog box!
First, as we want to aggregate the number of points within each grid square, we need to specify closed regions.
The extents of the grid (North, South, East and West) are filled in automatically from the box you drew on the map.
What requires thinking about is the grid size. How large should your grid squares be? This depends on a couple of things.
You may have to experiment a couple of times to create the appropriate sized grid cells. For this example, I chose to make the grid cells 5 miles square.
- How many points are you working with?
- How far apart are your points from one another?
- What is the size of your screen? Or the size of the map you want to create? Cells that are too large may make the map seem imprecise or "grainy". Cells that are too small may take a long time to create and use.
- What is the total area covered by your data set? 10 miles? 100 miles? 1000 miles? The larger the area, the larger your cells will need to be.
The grid cells in the map above are five miles square.
Once the grid is created, I like to color code the grid cells (using a Ranged thematic map). The grid cells will be colored with the number of points falling into each cell.
To understand the following:
The name of the table containing the grid cells is called Grid5mile
The name of the table containing the points is called Lots_of_Points.
See the below dialogs. The first is setting up the Join condition.
Now set up the join condition to count the points from the Lots_of_Points table.
Click the Join button. We need to perform a geographic join:
Side note: If Geographic joins are new to you, they were discussed in an article in last months issue.
Click OK. Once back at the Step 2 of 3 dialog box, check the option "Ignore Zeroes or Blanks...."
Ignoring the zeroes ensures that the cells outside of the area being mapped are excluded.
Click Next >.
We will modify the Styles to turn off the border for all regions. This is a handy tip in itself! Anytime you create a thematic map with lots of small regions, choosing 'N' (i.e. no line style) for the region borders will help to create a cleaner looking map!
Press the Styles... button. For each range color, set the border style to "N" (cell A1).
Once the thematic is complete, I like to finish up by using Layer Control to turn off the original point layer and the original grid layer. This will remove clutter from the map.
Here is the result!
As always, remember to save your workspace!
Article by Sue Disy, Senior Sales Engineer
When not writing articles for "The MapInfo Professional" journal, Sue works with Pitney Bowes Customers all over North America as a Sales Engineer. When not working Sue enjoys staying active. You may find her horseback riding in the Adirondack mountains, practicing yoga, enjoying personal travel or staying home and cooking in her kitchen.