3. Input Files, Parameters and Variables
3.3 Optional Files
3.3.2 The Block Definition File
The Block Definition File is very similar to the Conservation Feature File (see Section 3.2.2) and is used to set default variable values for groups of conservation features. It is always used in conjunction with the Conservation Feature File.
Groups of conservation features must first be defined using the variable, ‘type’ in the Conservation Feature File (see Section 3.2.2.2). Features may form part of a group because the values for some of their variables are, for example, defined by specific legislation, or they are similar in their ecological characteristics. For each different type of conservation feature, the values for all other variables in the Conservation Feature File can then be defined using Block Definition File. This provides a quick and easy method for implementing common targets for groups of features and is useful in cases where various targets are being used and different levels of protection explored. It also provides a quick way to provide proportional (percentage) protection, without having to manually calculate these values.
In order for groups of features to take on the values defined in the Block Definition File, the variable value in the Conservation Feature File must be set to ‘-1’.
An example of how values must be set in the Conservation Feature File (spec.dat) if you wish these parameters to take on the values defined in the Block Definition File (blockdef.dat). In this case, all 24 conservation features will take the Block Definition value for the variables ‘spf’ (Penalty Factor) and
‘sepnum’, and 11 of the conservation features will take the Block Definition value for the variable
‘targetocc’ (Target for Separated Feature Occurrences).
This is a convenient way to allow some variables to take on the group value and some to be set individually. For instance, in the example above the variable ‘target’ is defined for separately for each feature, perhaps because of differing abundance in the planning region. On the other hand, the variable ‘spf’, the Conservation Feature Penalty Factor (see Section 3.2.2.4), always takes on the value defined in the Block Definition File. This may be useful in cases where the different types of conservation feature are of differing importance but we are unsure of the appropriate ‘spf’ value. By using the Block Definition File we can alter the ‘spf’ for each type feature without having to alter every entry in the Conservation Feature File.
The Block Definition File can contain up to eight different variables and should be in the same format as the Conservation Feature File.
1 2
An example of the Block Definition File (blockdef.dat) used in Marxan. 1. Negative 1 must be entered in this file if you want the variable to take on the original value defined in the Conservation Feature File. 2. ‘Prop’ is the only new variable in this file --see below for description.
The only variable that is required in this file is ‘type’, and the only new variable is
‘prop’. Both of these are described below.
3.3.2.1 Conservation Feature Type Variable – ‘type’
Required: Yes
Description: A unique numerical identifier for groups of conservation features. Each
‘type’ must correspond exactly with the types identified in the Conservation Feature File (see Section 3.2.2.2).
3.3.2.2. Proportion Target for Feature Representation Variable – ‘prop’
Required: No
Description: The variable ‘prop’, is short for proportion and can be used to set the proportion (i.e. percentage) of a conservation feature to be included in the reserve system.
Getting Started: This should be a number between 0 and 1. For instance, if ‘prop’ for a type of feature is set to 0.2, then Marxan will set the target for all features within that type at 20% of the total abundance, based on the data in the Planning Unit versus Conservation Feature File (see Section 3.2.4). Total abundance is the sum total of the amount found in all planning units, including those that may be locked either in or
the value for the variable, ‘target’, should be set to ‘ -1’. It makes no sense to set both, and if both are set then the variable, ‘prop’, will take precedence and the variable,
‘target’, will be ignored.
The proportion is based on the total amount defined in the Planning Unit versus Conservation Feature File. If some pre -processing of the data has occurred, for instance to remove small or fragmented occurrences of a feature , then this may not be the same as indicated in the original data set.
3.3.2.3 All other variables
The remaining variables in the Block Definition File (‘target’, ‘target2’, ‘targetocc’,
‘sepnum’, ‘sepdistance’ and ‘spf’), all have the same definition as in the
Conservation Feature File (see Section 3.2.2) and where not defined here will take on the value in that file. If you wish any of these six variables, that are defined, to take on the original value from the Conservation Feature File, simply set the value in the Block Definition File to ‘-1’. This is also the default value for
these variables and any missing entries will take this value.
(This page intentionally blank)