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ii. In plotting small areas at this scale there are two major 
constraints (a) cartographic problems in plotting areas 
of less than 5 hectares (b) limitations in measurement: 
the smaller the features to be measured the greater the 
chances of inaccuracies. ‘This is highly important in 
order to achieve the standards of accuracy required in 
monitoring changes of areas as small as this. 
iii. The more detail contained on the mapped output, the more 
time consuming and therefore expensive the task of 
interpretation and the cost of measurement. 
Recording Base 
Tne recording base used was the Ordnacne Survey 1:50,000 series of maps. 
As with air photo interpretation techniques in other disciplines all 
secondary data sources immediately available were used to assist the 
interpreter. The sources of data, or 'ground truth' used in this survey 
to supplement the RAF photography were: 
om Ordnance Survey maps at a scale of 1:50,000. (124 map 
Sheets are required to cover England and Wales.) 
ii. Detailed street plans of the major urban centres. 
iii. Maps depicting navigable waterways. 
iv. Good maps of shopping areas. 
Production Method 
Before production began a series of technical meetings with DOE were held 
where a detailed examination of the specification was undertaken. 
As a preliminary to production, three format designs for use in the contract 
were approved by DOE all being of a simple layout to be used as overlays to 
the appropriate OS 1:50,000 map sheets. 
The format for use in the interpretation contained an overall 10 km grid 
which was used to correct for linear paper distortions of the map sheet. A 
reliability diagram was included in the margin for use in delineating areas 
of varying dates of photography. 
The formats for 'Developed Areas' and for 'Developed Areas and Main Urban 
Land Use Parcels' were of similar layout. Grid ticks only were supplied, 
the overall grid lines not being required on the reference and copying 
masters. 
The legend listing the five land-use types, the reliability diagram, scale, 
Sheet number, title and responsibility note were all positioned so as not to 
interfere with significant data on the OS map sheet when used as an overlay. 
Approximately 3800 1:60,000 contact prints were used to supply stereoscopic 
photographic cover of England and Wales. The sorties were broken down into 
the appropriate 1:50,000 map sheet and filed in boxes with a sortie diagram 
for that particular sheet. Each area was mosaiced to ensure complete 
coverage without gaps and cloud cover. About 90 per cent of the photography 
was flown in 1969, the gaps being mainly filled by other 1:60,000 photography 
flown between 1966 and 1975. In parts of North England larger scale 
photography was ued to infill gaps.