عـربــي


FREQUENTLY ASKED QUESTIONS (FAQ's)
 

How long does it take to make a map ?
Are computers used in map making ?
Now that there are satellite systems in space, making maps must be quick and easy !
When I check the map with my GPS receiver, I find that there are mistakes in the map.
 

 


 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.
 

back to top
 

 


 


 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.
 

back to top
 

 
 


 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.
 

back to top

 
 


 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.
 

back to top

 

 

 

 

 

National Survey Authority - FREQUENTLY ASKED QUESTIONS (FAQ's)