AUTOMATED GENERALISATION AND REPRESENTATION OF ORDNANCE
SURVEY POLYGONAL LANDCOVER DATA AT 1:10 000 SCALE
P.R. Revell
Ordnance Survey Great Britain, Research Labs, Southampton, UK
patrick.revell@ordnancesurvey.co.uk
Ordnance
Survey has very detailed polygonal data for landcover in Great Britain. The
classification includes: Coniferous Trees, Non-coniferous Trees, Scrub,
Orchard, Coppice, Heath, Rough Grassland, Boulders, Rock, Scree, Marsh,
Shingle, Sand and Mud. Within a polygon, combinations of these different
landcover types can occur, such as: “Heath + Rough Grassland + Scrub”. Each
valid combination has a feature code, and in total there are nearly 500
possible combinations.
Ordnance
Survey 1:10 000 scale specifications include less than 100 of these landcover
combinations, and no more than three landcover types are permitted within each
combination. Combinations which do not conform to the target specification are
highlighted and the cartographers reclassify them manually. A finishing process
automatically applies vegetation and rock symbol “wallpaper” patterns to the
polygons. The “wallpaper” does not look cartographically pleasing when the
symbols are broken by the perimeters of the polygons. Legacy hand-drawn rock
detail is also shown at 1:10 000 scale. The cartographers often have to
reclassify the polygon data to prevent “wallpaper” rock symbols from
conflicting with the hand-drawn rock.
This
paper presents research into a user interface which allows landcover
reclassification rules to be developed and verified. A reclassification rule
comprises a condition, followed by a list of landcover types to remove (and
occasionally a list of landcover types to add) if the condition is fulfilled.
The interface is not just for developing rules for the current 1:10 000 scale;
it also allows new specifications and rules to be designed for any target
scale. The interface additionally facilitates integration of third-party
landcover data with the source polygonal data, for example hand-drawn rock. The
output from the interface is a reclassification table which forms an input to a
model generalisation process.
Following
reclassification, adjacent polygons may have the same feature code; therefore
the following step is to merge such polygons. Holes below the minimum size for
1:10 000 scale are subsequently removed. Next, polygons below the minimum size
are either merged with the neighbour with the largest area or the neighbour
with the largest shared boundary. Lastly the polygons are simplified whilst
maintaining the topological consistency of shared boundaries.
The
final step places vegetation and rock symbols automatically according to the
classification of each polygon. The symbol placement algorithm is designed to
avoid conflicts between the symbols and the polygon boundaries. The result is
of a high cartographic quality and is achieved with no manual intervention.