What are Grid items?

Grid items are similar to Bitmap items.

In bitmap items each pixel stores a colour. In Grid items however each pixel or cell stores a numerical value.

The cell values are mapped to colours by using a Colour-set object.

Grid items could be used to store population densities or land-use.

• For population densities, the value of each Grid item pixel would be the number of people who live in the square that the pixel covers.
• For land-use, each pixel would store a code number for the usage of the land at that point, so 0 could be agricultural, 1 could be urban, 2 could be unused and so on.

A common use of Grid items is to store the height above sea level of the ground. Here the Grid item is often called a Digital Terrain Model (DTM).

Note: The pixels in a Grid item are actually 16-bit signed integers. However, each Grid item also stores a floating point Grid value resolution. This is applied to each pixel integer to yield the notional cell value. The choice of Grid value resolution is made automatically to maximise the accuracy of the Grid item.

1. Create a Grid item using existing items

Select several items and use Scatter Grid (Analysis > Grids).

The items which are used to create the Grid are often Point items. The simplest way to use Point items to make a Grid item is to count them.

This will result in each Grid cell storing a count of the number of Point items that were in that cell.

2. Create a Grid item from other Grid items

You can create a Grid item by combining other Grid items with an expression.

Begin by saving copies of the other Grid items in a Named Object Library. This allows you to then enter an expression that refers to those Grid items by name.

So if you have saved two Grid items as G1 and G2 you can then use these with the following expression:

FindGrid("G1")*FindGrid("G2")

Using Grid items

To examine the value of a grid item, choose the Grid item (e.g. DTM or DEM) option in the Status Bar.

This gives you the value of the Grid item cell under the cursor as you move the mouse.

Use Analysis > Grids > Create Shadow to create a monochrome bitmap indicating relief by shading. This is often used for Digital Terrain Models (DTM) to give a cartographic effect.

The shadow is calculated by examining the difference in value between adjacent Grid item cells. This corresponds to the gradient of the ground at each point.

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