Ministry of Environment Wildlife

User's Guide for ERM Mapping Tool

Creating Simple ERM Themes

Read here about menu options that permit creation of themes.

Simple themes can be created in a variety of ways. The two types of simple themes are suitability and capability. Suitability and capability can be displayed using five methods; averaged, dot-density, highest value, largest area, and largest area breakpoint.

During initial creation of a theme, only averaged, highest value, and largest area display methods are available. Dot-density and largest area breakpoint are available after a theme is created by selecting the 'Symbolize ERM Themes' menu option.

Averaged

With averaged, the ratings for each component are averaged over a polygon. For example, if the six-class scheme is being used, and if a polygon is composed of 50% with a rating of 2 (midpoint = 63), 30% with a rating of 4 (midpoint = 16), and 20% with a rating of 1 (midpoint = 88), then the final value assigned to the polygon is calculated as follows:

final value = ( 5 * 63 + 3 * 16 + 2 * 88 ) / (5 + 3 + 2)
final value = 54 %

where:
Red numbers refer to the decile of each component of the polygon, and
blue numbers refer to the percentage representing the midpoint of each rating class.

Because 54 % falls within class 2, the polygon will be coloured with the colour representing class 2.

Using Averaged to display habitat will tend to hide high value habitats. However, averaged gives a good quantitative measure of the amount of habitat within a polygon. Figure 1 shows an averaged theme and an associated Identify report for a polygon.

Figure 1. Averaged theme, with associated Identify report, for the polygon under the mouse pointer.

Highest Value

With highest value, the value assigned to a polygon is equivalent to the percentage that represents the rating of the component with the highest rating. For example, if the six-class rating scheme is being used, and if a polygon is composed of two components, and the deciles for each component are 6 and 4, and the ratings for each component are class 4 and 2 respectively, then the percentage assigned to the polygon will be 63%. The polygon will be coloured with the colour representing 63% (i.e. class 2).

Figure 2. Highest Value theme, with associated Identify report, for the polygon under the mouse pointer.

Largest Area

With largest area, the value assigned to a polygon is equivalent to the percentage representing the rating that covers the largest area. For example, if the WHR standard four-class scheme is being used, and if a polygon is composed of three components, and the deciles for each component are 4, 3, and 3, and the ratings for each component are H, L, and L respectively, then the value derived for the polygon will be 13% (midpoint of class L). The polygon will be coloured with the colour representing L because 60% of its area is rated as a Low.

Figure 3. Largest Area theme, with associated Identify report, for the polygon under the mouse pointer.

When a theme is created, a dbf file is created in the tool's working directory. The file is named with the following convention:

• <spatialdata_name>_theme_<theme_id_number>.dbf

For example, a dbf file name might be 'tecp_lig_theme_1.dbf', where tecp_lig matches the name of the spatial database, and the '1' indicates that this is the first theme created. You should not modify these tables, because they are managed by the tool.

After a theme is created, it is automatically opened, and added to the top of the theme list in the view.