When you are working in the BAP Priority Habitats or Farmland Birds streams,
having selected your Primary Information layer you can then create a Thematic Map layer. This allows you to
show features from your chosen Primary Information layer (for example, Government Regions) shaded according
to their importance for a BAP priority habitat or Farmland Bird species. For BAP Priority Habitats the
percentage of the national resource present in each feature is used as the basis for classification;
for Farmland Birds, the assigned National Ranking value for each feature (based on the total proportion of
all 'Top Sites' that it has been found to contain) is used.
Having chosen a habitat or species to thematically map, you then need to select a
classification method. For BAP Priority Habitats, and for most Farmland Bird species, there are two choices -
Equal Interval (5 bands) or Quintiles.
The Equal Interval (5 bands) classification ensures that the range of possible values
(excluding zeros), is divided into equal-sized intervals, although the number of features in each class will
vary and the setting of class limits is done automatically from the range available. Features containing none
of the chosen habitat are treated as a special case exempted from the classification, and displayed as
transparent polygons (no fill) - they are listed "0" in the Legend. So for example if the range of percentage
values for your chosen Primary Information layer is 0 to 50.00, the symbolised classes will be "0"
(transparent fill), with five Equal Interval classes set at approximately 0.01 - 10.00, 10.01 - 20.00,
20.01 - 30.00, 30.01 - 40.00 and 40.01 - 50.00. You can choose to display the 5 classes or bands in a series of
reds, greens, blues or greys.
The Quintiles classification ensures that each class has the same number of values (one
fifth of the total number, excluding zeros again) from the Primary Information layer dataset. This will produce
unequal class ranges but classes at the extremes and middle will have the same number of values. Information
layer features with none of the chosen habitat are again exempted from the classification, and symbolised
as transparent (no fill) - and again they are listed "0" in the Legend. You can choose to display the 5
classes or bands in a series of reds, greens, blues or greys.
There are a few important exceptions to this behaviour, relating to the Farmland Birds
Thematic Map layers. Classification of Farmland Birds data is based on National Ranking values. Where these
values have a large range eg 1 to 40, classification in to groups is quite straightforward. Some Farmland Bird
species, however, have less than five discrete National Ranking values and cannot therefore be classified
adequately using 5 Equal interval bands, or Quintiles. In these situations a "Unique Values"
classification is used, ie one class is assigned to each National Ranking value. For Equal Interval and
Quintile classifications for the Farmland Birds data generally, the more rank values present, the more
intuitive and understandable the classification will be. Classifications based on only a few values will,
however, still serve to differentiate higher ranking from lower ranking areas, and the use of the Information
Tool will show the rank values for individual features.
The other exception to the normal Thematic Map layer behaviour relating to Farmland Birds
is where 'Important Sites' is selected as the thematic option for a Farmland Bird species; in this case the
thematic classes are fixed as 'Top Sites' and 'Other Sites'. See
What's in the Targeting and Planning Map: Farmland Birds stream.
In the BAP Priority Habitats stream, the classified features will also be labelled when
you are zoomed in enough, to indicate their ranking. For example, the label "Rank: 23 of 330" for a
feature indicates that it ranks 23rd in terms of the percentage of the national resource for that habitat it
contains, out of a total of 330 Information layer features which contain at least some of that habitat.
