The main set of around 378 pillars across the country.
In most areas of Great Britain and its islands, the primary stations were built before any lower order markers. Out in the field, the clue to these pillars being older is where primary station pillars normally contain an older flush bracket that bears a much lower number than the surrounding lower order pillars.
Once the primary network was in place, imaginary lines were projected between many of these primary stations, from one to the next. These imaginary lines then became the edges of secondary blocks, making way for the next order of triangulation to proceed.
Secondary pillars were positioned to fill in the gaps between the primary pillars.
With a network of imaginary lines connecting the nearest primary stations already in place, secondary stations were set up in clusters within the areas marked by the primary lines. These Clusters were known as Secondary blocks.
Neighbouring secondary blocks were often built quite separately from one another. This is why baggers will often find a series of consecutive flush bracket (serial) numbers in one area. When out bagging, if there is a sudden change from the range of FB numbers already encountered, there is a good chance that a primary projection line has been crossed and the bagger has entered a different secondary block.
Secondary stations were surveyed to the same accuracy as the primary stations.
Third order (tertiary) stations filled in the gaps between the second order stations.
The majority of triangulation pillars in Great Britain and its islands are tertiary stations. Most tertiary station trig pillars were built concurrently with their neighbouring secondary station pillars. This means that within a given secondary block, the flush bracket numbers on tertiary pillars are mixed in the same range as the secondary pillars.
Fourth order stations filled in the gaps between the tertiary stations. Fourth order pillars were often added to secondary blocks later than their secondary and tertiary neighbours. Other stations which are now fourth order are sites which have been demoted from tertiary or secondary, possibly due to ground movement.
13th Order (GPS)
The marks for GPS Stations, which were marks surveyed by GPS before the Passive Station Database was created, are sometimes referred to as 13th order.
A table of marks, showing the different types and their relative frequency, is available.