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FREQUENTLY ASKED QUESTIONS (FAQ's)
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How
long does it take to make a map ?
The time can range from a
few months to perhaps a couple of years, depending on the complexity
of the task, the number of individuals working on the map, and the amount
of information to be shown.
Topographic maps contain
an enormous amount of detail about both the natural and the man-made
features that exist on the earth’s surface. As well as such familiar
features as roads and buildings, place names and coastlines, maps will
show types of vegetation, types of surface materials (sand, gravel,
sabkha, wadi spread, rocks), tall structures (electricity pylons, masts,
high buildings) and elevation features (spot heights, hill shading,
contours). A look at the legend of a Series K6611 or 50-04 maps
will show the extensive range.
All this information has
to be collected, drawn and positioned accurately in its proper place,
and checked thoroughly to make sure that it is correct in every respect.
This all takes considerable time, skill and patience. Only when
this is complete can the map be printed.
The map production process
starts with the taking of vertical aerial photography using a specially
designed camera with a very precisely manufactured lens mounted in a
fixed-wing aircraft. It is worth noting that, in general, satellite
images are not sufficiently accurate for the scales of topographic mapping
produced by NSA. Field surveyors relate these aerial photographs
to the ground by establishing an array of ground control points in a
rigorous survey. It is the task of Photogrammetrists to extract
the information required for the map from these controlled aerial photographs.
A familiar phenomenon of aerial photographs is that the tops of tall
structures and high features spread out from their bases. Clearly,
this is no good for precise measurements as the tops and bases of these
features should coincide. Photogrammetrists use special instruments
that create a truly vertical 3-dimensional view that compensates for
tips and tilts in the photographs and the displacement of features due
to their height. The information collected by the Photogrammetrists
is then passed to the Cartographers who draw and symbolize this information
to the required specification. The Reprographics specialists produce
a proof copy of the map for checking. This will be checked for
completeness, accuracy and currency, and will involve the Field Surveyors
in verifying that the map reflects the details on the ground and adding
any changes that have taken place since the photographs were taken.
The Cartographers will carry out the corrections and add any update
information before passing to the Plate-makers to make the printing
plates for the Printers to print the final map stocks, using offset
lithography techniques.
Computer-assistance and automation
of many of the traditional manual processes is helping to speed up the
time it takes to make a map. However, the time consuming processes
of deciding what to put on the map, of collecting this information from
the aerial photographs, and checking to make sure that everything is
correct can still only be undertaken by a human being. In summary,
high quality map making is still very much dependent on the training,
skills and experience of specialist individuals.
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Are computers used in map making ?
Yes. The complete map-making
process has become digital. All individuals who are working in
the map-making profession these days have to be competent computer users.
The computer hardware has to be the very latest in terms of performance
and capacity, and the computer software is both highly specialized and
sophisticated. Field surveyors record their information directly
from GPS receivers and electronic survey instruments on automatic data
loggers that replace pencil and paper. Photogrammetrists use analytical
and digital instruments in place of mechanical instruments to collect
information from aerial photographs, often in digital form, in strings
of digital data. Cartographers use top-end computers with sophisticated
software to prepare the digital data to conform to the customer’s specification,
to prepare 3-dimensional views of the ground, and to carry out map publishing
tasks to produce a wide variety of paper and digital map products.
Reprographics specialists operate very high resolution, large format
scanners to create digital representations (softcopy) of paper products,
and film writers that produce final hardcopy products from the digital
data.
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Now that there are many satellite systems
in space, making maps must be quick and easy ?
No, this is a myth
as far as the topographic maps produced by NSA are concerned.
In general, the images produced by remote sensing satellites are neither
of sufficient geometric accuracy nor of a sufficient scale to be able
to determine the exact details of a feature. Nor do satellite
systems generally provide 3-dimensional information to produce contour
and height values, and to eliminate the errors due to atmospheric conditions
and the non-verticality of the imagery. The extent of follow-up
verification of the information taken from the satellite imagery by
Field Surveyors is considerable. Mapping-quality aerial photographs
continue to be a superior source of accurate information for medium
and large scales topographic map production.
There is no doubt that
remote sensing satellites have a valuable role to play in certain aspects
of map-making. They are a vital source of information where high
accuracy is not required, such as for land-cover and environmental studies,
for pollution monitoring, for small scale mapping, for thematic mapping
and for geological studies. The newest mapping satellites have
resolutions of better than 1 metre (an incredible level of performance
that is suitable for mapping at scales approaching 1:2,400) but the
cost of the data is extremely high, so limiting its use to very small
areas for specific studies.
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When I check the map with my GPS receiver,
I find that there are mistakes in the map ?
It is unlikely, though
statistically possible, that there are mistakes in the map that can
be detected by a hand-held GPS receiver. A map does not claim
to be 100% correct. In reality, it never can be. Indeed,
many maps quote an accuracy statement for the information on the map,
usually in terms of a statistical figure such as “90% of the features
on the map are within 10 metres of their true position”. This
of course implies that as many as 10% of features can fall outside this
value. There will also have been changes in the map details since
the map was published. Sadly, a map is always out-of-date from
the moment it is printed.
What a GPS user is likely
to have experienced is a wrong set-up in his GPS receiver relative to
the map he is using. It is essential that the GPS receiver be
set up to conform to the map. In particular, the co-ordinate system
and its datum have to be the same. Most of NSA’s maps are constructed
on the World Geodetic System 1984 (WGS 84) spheroid and datum, and use
the Universal Transverse Mercator (UTM) projection to project the position
of features from the curved, 3-dimensional surface of the earth to the
flat, 2-dimensional surface of the map paper. Nevertheless, users
will encounter maps of Oman that use other datums and projections, such
as the European Datum 1950 (ED50) and the Lambert Conformal Conic projection.
The details for each specific map will be found in the margin information
on the map, and must be entered correctly into the GPS receiver if an
accurate relationship between the GPS receiver and the map is to be
assured.
If users do not have
the specific WGS 84 datum in their GPS receivers, WGS (or WGS 72) may
be used with no discernable loss of accuracy for virtually all situations.
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