Distribution maps are created by recording which species are found within each square of the national grid. The first Atlas was produced by the BSBI in 1962, showing black dots for records since 1930 and open circles for older records.
Since then there have been numerous similar maps published, most notably Scarce Plants in 1995, and the New Atlas in 2002. These plotted current records (‘black dots’) as those made after 1970 and 1987, respectively. Thus there are four traditional date classes for recording plants, as follows:
- DC0 = -1929
- DC1 = 1930-1969
- DC2 = 1970-1986
- DC3 = 1987-1999
- DC4 = 2000-2009
- DC5 = 2010-2019
From now on we plan to record in decade-long date classes, so Date Class 6 will be 2020-2029. Another innovation is the change from 10km squares to 2km squares for recording units, which is increasingly coming into use; but, although these use the same date classes, those are not mapped because it would be impossible to achieve realistic levels of coverage.
Our aim is to get tetrad maps of all species by 2020. At the moment they are somewhat incomplete.
A tetrad is a 2km x 2km square, given a letter code as shown below. So tetrad SW41A is made up of the four 1km squares SW4010, SW4011, SW4110 and SW4111.
-- The DINTY system for tetrads --
Getting grid references to display
Jim McIntosh sends these instructions for getting the grid references to display in your browser when using Maps Scheme maps.
Can I get the data behind the Maps Scheme?
The data are highly simplified and structured for the purposes of the Scheme. There are no more details in the database, so if you want to find precise locations, dates, etc., you have to trace the record back to its original source. For this you normally have to contact your county recorder.
What do the maps mean?
Distribution maps need to be used with a level of caution. Especially for the more difficult species and hybrids, the maps can often reflect the places where a few skilled recorders have been, rather than the true distribution of the plants. Also, levels of recording in each date class are not equivalent. Finally, there is much confusion between DC0 and DC1 in the database, which needs further work.