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Most 
y has 
been in equatorial regions where clear 
water allows detection of underwater 
features. High levels of suspended 
sediment in many coastal waters effectively 
prevents any information about water depth 
being extracted. 
Thirdly, atmospheric contamination, and in 
particular cloud is often present and 
significantly reduces the amount of useful 
data that can be obtained from any 
particular image. 
Finally, tidal considerations are very 
important as far as hydrographic 
applications of imagery are concerned. The 
vertical datum of Admiralty charts is LAT 
and all hydrographic features are charted 
relative to this. The fixed orbits and 
repeat cycles of satellites mean that it is 
unlikely that an overpass will coincide 
with a low tide. Tidal height for 
particular images can however be accurately 
calculated and the usefulness of future 
images can be predicted. 
5. CASE STUDIES 
To date, imagery has been used to improve 
detail in some way or other on about 60 
Admiralty charts. A number of examples are 
presented here in order to illustrate this 
practical application of remote sensing in 
a production environment. 
The most common use for satellite imagery 
in the Hydrographic Office is to aid the 
compilation of a new chart for a region 
where there is a paucity of source data. 
The imagery is used to verify existing 
chart detail, to identify major errors in 
the depiction of coastline, and to identify 
islands and reefs which are uncharted or 
out of position. Landsat Thematic Mapper 
imagery is being used in this way to 
provide data for a series of new charts 
covering the Red Sea. Figure 1 illustrates 
the area around Al Qunfidha. Although a 
small portion of the current Admiralty 
Chart is based on a 1918 survey. the 
majority of this area has never been 
adequately surveyed. Figure 2 shows the 
chart overlaid with reefs, shoals and 
coastline derived from Landsat TM imagery. 
Many of these features are either uncharted 
or significantly out of position. Even in 
Home Waters source data may on occasion be 
lacking. For example a SPOT scene has been 
used to provide details. of channels and 
sandbanks in the eastern part of the Solway 
Firth to assist in the compilation of a new 
edition of chart 1346, published in 1989. 
This area had never been previously 
surveyed and on the former edition of the 
chart very little detail was shown. 
A second use may arise when a Hydrographic 
Note is received from a mariner reporting 
an error or omission from a chart. Imagery 
may then be used to verify the report and 
to delimit and position the feature to 
enable it. to. be . correctly .. charted. 
Lighthouse Reef near Belize in Central 
America provides a good example. An 
extract from a former Admiralty chart of 
the area, current as recently as 1988, is 
shown at Figure 3. 
509 
Although modern aerial photography was 
available covering the entire reef there 
was no way of accurately locating it in 
relation to the mainland and ships' reports 
had indicated that the reef was mischarted 
by up to 1 mile. The old chart was based 
on 19th century surveys. A stereo pair of 
black and white Spacelab photographs was 
available and absolute orientation based 
upon 10 control points yielded planimetric 
accuracies of around 40 metres. The pecked 
line shows the reef edges as derived from 
the Spacelab photography. Not only has 
this photography enabled Lighthouse Reef to 
be accurately positioned, it has provided 
a means of controlling the larger scale 
conventional aerial photography which has 
been used for detailed plotting. A new 
chart, 959, covering this area at 1:125 000 
was published in 1989. 
Thirdly, satellite imagery may be used to 
resolve problems arising from conflicting 
information. A SPOT scene has enabled the 
main entrance to Saint Louis, a port in 
Senegal, West Africa to be accurately 
charted. The best available alternative 
data shows the entrance to be over 2 
nautical miles north of its true position. 
In another recent example Space Shuttle 
photography (Large Format Camera) has been 
used to confirm the correct positions in 
the vicinity of Corfu. 
6. CONCLUSION 
Photogrammetry, as an integral part of 
coastal hydrographic surveying and 
charting, is a powerful and versatile tool. 
Satellite Imagery in its various forms 
provides a significant and cost effective 
means of supporting small scale nautical 
chart compilation and revision. In areas 
of clear water where existing surveys are 
inadequate, imagery can provide the basic 
data to be included in new charts.