While photogrammetry does not in any way 
replace the hydrographic surveyor it has 
most to contribute in those areas which are 
dangerous, difficult or time consuming to 
survey by traditional methods. 
An accurate photoplot which shows all 
photographically visible dangers in the 
nearshore zone reduces the requirement to 
survey here and hence risk damage to the 
survey vessel. Drying areas can only be 
surveyed at limited times and adverse 
weather can further impede progress. The 
use of photogrammetry, once the initial 
photographic coverage and ground control 
have been obtained can progress the 
plotting work without interference from 
external influences and at a rate which can 
be reliably estimated. In addition and 
significantly, modern photogrammetric 
techniques can achieve accuracies on a par 
with hydrographic surveying practices. 
The use of photogrammetry thus enables the 
surveyor to concentrate his efforts on the 
deeper water areas. However, even in the 
inshore zone some measure of field 
surveying is always necessary since there 
are limitations on applying aerial survey. 
The photography will seldom, if ever, be 
flown at a tidal state corresponding with 
chart datum. A combination of good weather 
and low tide is notoriously difficult. to 
come by. As a result the waterline on a 
photogrammetric plot will inevitably be 
some distance landwards of the true drying 
line at Lowest Astronomical Tide (LAT). 
Water clarity is seldom ideal particularly 
in the sediment-laden waters of the UK. 
Best results are obtained in rocky areas 
such as the Hebrides, but it is rare to 
achieve measurement of water depth greater 
than 10 meters. Under such circumstances 
the fact that navigation dangers cannot be 
seen does not imply that they do not exist. 
Photography taken over water can lead to 
problems of sunflare if the sun angle is 
greater than about 40 degrees and large 
water areas can make photography very 
difficult to use with a traditional analog 
stereoplotter. Extensive drying areas may 
be lacking in texture on photography making 
good stereo interpretation difficult. The 
completeness of the photogrammetric survey 
of such areas may be limited accordingly. 
4. USES OF REMOTE SENSING 
Until the mid 1980's the usefulness of 
satellite data as a cartographic tool was 
seriously limited by poor resolution. 
However three forms of high resolution 
imagery are now commercially available and 
are regularly employed in a charting 
context. These are Landsat Thematic Mapper 
(TM), SPOT (Multispectral and Panchromatic) 
and Spacelab photography (Metric Camera and 
Large Format Camera). 
These various types of satellite imagery 
provide valuable data for charting at 
scales of around 
1:75 000 and smaller. Satellite imagery is 
not seen as providing a definitive survey 
but rather as a cost effective source where 
alternatives are unavailable. Imagery can 
be particularly useful in the planning 
508 
stage. of ..a survey. For example in 
developing a new port possible deep water 
routes through inshore . reefs can be 
detected. The hydrographic surveyor need 
not spend time in surveying and exploring 
possible . routes which in the event are 
unlikely to permit the safe passage of 
vessels. 
A major advantage of space imagery over 
conventional aerial photography is the vast 
area covered on one scene. This enables 
offshore islands and reefs at a 
considerable distance from the mainland to 
be accurately positioned. The spectral 
discrimination provided by the various 
wavebands of Landsat and SPOT imagery is 
also an advantage. The near infra-red 
provides a clear coastline whereas the 
blue/green waveband permits identification 
of underwater features in depths of up to 
20 metres in ideal conditions. 
Satellite imagery provides a valuable means 
of detecting coastal and shallow water 
features where their previous existence was 
unknown. This is of particular 
significance in remote areas where imagery 
can also help to improve the delineation of 
features sketched by early surveyors. 
At present much of our work involves the 
use of hard copy, products in film positive 
format. The geometric accuracy of both 
SPOT and TM photographic products has been 
found to be very good. In areas of 
reliable coastline the imagery consistently 
gives an excellent fit to a Transverse 
Mercator projection. Considerable success 
has been achieved in identifying and 
correctly positioning coastline and 
submerged dangers to navigation. In the 
absence of known depths it is not possible 
to . provide. absolute . values | but the 
identification of shoal areas in poorly 
surveyed waters is of significant value to 
the charting task. 
The use of digital SPOT and Landsat data is 
also currently under investigation. Known 
depths are used for calibration with the 
aim of deriving reliable  bathymetric 
contours in areas where surveyed data is 
unreliable or non-existent. 
Spacelab photography, both Metric Camera 
and Large Format Camera, is processed using 
traditional photogrammetric techniques with 
successful results. 
The information available from satellite 
sensors is 
therefore of considerable importance to the 
charting of remote areas where alternative 
sources are inadequate or non-existent. 
However, there are a number of factors 
which limit the use of remote sensing and 
which fall into four main categories. 
The first of these is image resolution. 
For example the spatial resolution of SPOT 
with a pixel size of 10 by 10 metres is 
such that small pinnacle features which 
could be extremely hazardous may not be 
detected. 
Water clarity is a second limitation. Most 
of the work done to date with imagery has