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Scatter Grid Scatter Grid
Scatter Grid uses a wizard to create a Grid item from several selected items. The Grid item will be large enough to just cover the origins of all of the selected items.
The items used to create the Grid are often Point items. The simplest way to make a Grid using Point items 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.
You can choose the resolution of the Grid item (i.e. the size of the Grid cells) within the Scatter Grid wizard. You are able to create either rectangular or square Grid cells, either way the cell size must be specified. The size of the Grid cells should be determined by the extent and type of the data being analysed. The larger the cell size (lower resolution) the more generalized the Scatter Grid will be.
The wizard also allows you to choose how the selected items are used to determine the values of the Grid item's cells. You can either simply count the Point items, or you can do various operations using a property or expression on each Point.
The Grid will be created as an item and can be stored either in an existing editable overlay or within a newly created Internal Overlay.
Sometimes you have data stored as properties of Point items, and you wish to convert this data to Grid format. To do this you can use Scatter Grid and specify the Point items’ property name. So for example, if the Point items were drawn in 3D, then you could use the Origin Z property.
When you use a property of the Point items you should specify how to combine the property values, see the Create Grid from Points dialog:
Example:
Select the items on which the grid is to be based.
Click on Scatter Grid [Analysis-Grids].
The Grid Creation dialog will be displayed:
Cell Size
Allows you to set the size of the Grid cells, in metres. By checking the Square tickbox the Height value will be uneditable and equal to the Width value, producing square grid cells. If the Square tickbox is unchecked rectangular grid cells will be created. Cell size is also known as the grid resolution (Resolution property). The Grid size resulting from the Height and Width values is shown.
Origin
Checking the Snap to cell size tickbox will round the X and Y co-ordinates so they are multiples of cell width and height. With Automatic selected the origin of the Grid is set automatically based on the extents of the selected data. Specific makes the X and Y values editable, allowing you to manually specify the origin.
Preview window
Throughout the wizard the preview window will show the Grid to be created according to the setting chosen. Click in the preview window and hold down the middle scroll button to enable panning, click and roll the middle scroll button to zoom.
Note: Smaller Cell Size values will create a higher resolution Grid, but the Grid will take longer to generate. Therefore clicking Next to progress through the dialog will take longer than with larger Cell Size values.
If the dialog fails to progress or does not show a grid after clicking Next it is possible that the grid cell size is too small. Go back to the Grid Creation dialog and alter the grid cell size.
After selecting the Cell Size and Origin of the grid click Next.
The Create Grid from Points dialog will be displayed:
Note: This example SWD includes a schema called Pollution_Index&
Check the Count tickbox to create a Grid where each cell contains a value of the number of items (often points) contained within that cell.
Uncheck the Count tickbox to enable the Property/Expression field, which is directly used with one of the four grid creation methods; Sum, Interpolate, Closest point and Barnes interpolation.
Property/Expression
Press the fx button
to display the Pick a Property or Expression Builder dialog.
Select either the Property or Expression to be used. This example uses the schema called Pollution_Index&.
Sum
The item property values within each cell are added together and stored within the corresponding Grid cell.
Interpolate
The Grid cell values are interpolated from surrounding Point items. This method could be used where the property gives a continuously varying measurement, such as height above sea level or polution levels as used in this example.
Closest point
The Grid cell value corresponds to the value of the closest point.
See Interpolation in SIS for a full description of the Interpolate and Closest point options.
Barnes interpolation
Constructs a grid of size determined by the distribution of the two dimensional data points. Using this grid, the function values are calculated at each grid point.
Grid Values
Use No-Data values for zero values
Check this tickbox to treat Grid cells containing zero values as 'No-Data' and therefore making them hollow.
On completion of the Create Grid from Points dialog click Next.
The Style the Grid dialog will be displayed:
Statistics
This section gives an output of the Grid statistics, which include the Minimum and Maximum cell values, the Average cell value and the Standard Deviation of cell values of the grid.
Colourset
Values
Multiples of Average
This option is automatically selected and sets the number of intervals according to the Grid cell Average. The range of each interval will equal the Average. With this option the Values drop-down box will be unavailable..
Multiples of Average is automatically selected and sets the number of intervals according to the Grid cell Average. The range of each interval will equal the Average.
Multiples of Std Dev
This option sets the number of intervals according to the Grid cell Standard Deviation. The range of each interval will equal the Standard Deviation. With this option the Values drop-down box will be unavailable.
Evenly from Min to Max
Sets the intervals to equal sizes. The Values drop-down box will become active to allow you to select the number of intervals.
Note: Predefined coloursets can only be applied when less than 13 intervals are chosen.
Once the Colourset > Values selection has been made you can either click Pick Colour... to choose individual colours or click Pick all Colours... to pick a colourset.
Displays the standard Windows Color selection dialog to allow a basic or custom colour to be selected.
This button is active when Evenly from Min to Max is selected with at least 3 values.
Pressing the Pick all Colours button will display the Pick Colours dialog which provides a number of predefined coloursets.
ColorBrewer can be expanded to show the predefined coloursets. See the ColorBrewer topic .
If you have a range that does contain data but is at a low level that you do not want to be visible in the Grid you can select the range and set the value to fully transparent, for example:
Hue (range 0 to 240) this is the wavelength of the colour. This corresponds to a position in a rainbow of colours. The value changes the colours from red (at 0) to green to blue, and back to red (at 240).
Saturation (range 0 to 240) this is the purity of the colour. For example, pink is an unsaturated form of red. Primary colours are saturated, pastel colours are unsaturated.
Luminosity (range 0 to 240) this is the brightness of the colour. Very dim colours become black.
Transparency (range 0 for fully opaque to 255 for fully transparent) this value sets the degree of transparency.
A quick way of creating a new colourset is by changing the colour of the first and last intervals, selecting all the intervals between and clicking Spread.
The Spread button gives the following options:
The first interval colour will be merged into the final interval colour across the selected intervals. The following image shows an example of this:
On completion of the Style the Grid dialog click Next.
The Review the Grid dialog will be displayed:
The Review the Grid dialog allows you to select where the Grid item will be created. If you want to store the Grid item in an existing Overlay, select this Overlay in the Add to an existing Overlay pane. This Overlay must have a status of Editable and be able to store Grid items. Alternatively you can select Create in a new Internal Overlay, which will create the Grid item in a new overlay, in this case enter the name of the new overlay in the text box.
You can style the Grid using a Grid Colourset or apply a Relief Theme.
Grid Colourset
Select this option to style the grid using the colourset property previously selected in the Pick Colours dialog. The styling cannot be stored, however it can be changed by right clicking on the Grid and selecting Pick Colours.
Relief theme
Styling is used to create a Relief Theme and does not affect the Grid item itself. This Relief Theme is placed over the top of the Grid and can be turned on and off. The Relief Theme can be stored as either a new Theme or Colourset to be used again on another Grid. Once it has been created a Relief Theme cannot be edited. If the Relief Theme is turned off the Grid itself will be coloured according to the Overlay pen, with differing colour intensities showing changes across the Grid.
Whichever styling option is selected here, once the Grid is created the Grid Colourset can be changed to alter the colouring and the number of intervals. This is accessed by selecting the Set Colourset option in the local menu, this displays the Style the Grid dialog (shown above).
On completion of the Review the Grid dialog click Finish to create the Grid item.
Example of a Grid item displayed in the 3D Window with height exaggeration Exaggerate heights [3D-Display] applied:
It is recommended that initially the analysis is run several times, altering settings and styling, so that the created Grid is optimised for its purpose.
If the Grid is stored within an internal overlay it can be found under 3D as Grid:
The outputted example Grid is displayed below in 2D and overlaid on top of an ECW image. If the Use No-Data values for zero values option (Create Grid from Points dialog) was selected those no data areas can be set to hollow, showing underlying base maps. This is done by selecting the Grid overlay properties, press F2 to display the Overlays dialog and navigate to the Styles tab, where the Brush is set to Hollow, in the Edit Brush dialog, and Override is selected in the Overlays dialog - Styles tab. Transparency of the Grid can also be set in the Styles tab with the Transparency Overrides > Map Window Bitmaps slider.
The Grid can also be viewed in 3D, allowing trends to be easily viewed.
Note: To view in 3D the overlay Brush property must not be set to Hollow.
Note: If there are problems viewing the grid, ensure that the projection/coordinates options are correct and that the grid is deselected. If you view your grid in 3D, you may find it does not look as you would expect, lowering the exaggeration to less than 1 may correct this.
See the following sections relating to grids for further details:
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