Because of the small scale the map could not be very detailed. Most 
categories were possible to distinguish on the aerial photographs. 
Industrial areas. for instance. are often easy to distinguish - large 
buildings with flat roofs, stored goods in the yards. Pure residential 
areas are also easily identified by gardens and parking lots. We had 
some problems in separating city center from the "mixed'area surrounding 
it. This could mostly be done by studying the yards. Blocks containing 
dwellings often have some kind of playing-ground or other facilities for 
the tenants. Schools could be identified by playing-grounds and gym 
halls (Fig 2). 
Dwellings were sub-divided into six categories: 
Sparsely-developed single-units 
Densely-developed single-units 
Low-rise slab blocks (less than 5 storeys) 
High-rise slab blocks (5 storeys or more) 
Tower blocks 
Continuous development. 
This subdivision was mostly fairly easy to make. The only problems were 
to separate sparsely-developed single family houses from densely-developed 
ones and low-rise slab blocks from high-rise blocks. However; this could 
be done together with the photogrammetric density calculations (Fig 3). 
To determine development density, we studied dwelling areas and divided 
them into areas containing houses of the same category (Fig 4). The 
picture shows a part of Sátra in the north-west of Gávle. Two areas with 
sparsely-developed single-units; two with densely-developed single- 
units, three with tower blocks and one with low-rise slab blocks have 
been marked. 
Each area was determined by X and Y coordinates and traced by a 
coordinatograph on a map. 
In areas with identical buildings; the houses were counted and the 
coordinates of one house were determined. In heterogeneous areas all 
houses were measured. The area surrounding the city center, for 
instance, was treated block by block. Big single-unit areas were