Saharan Geo-data

With the increasing availability of technology, many have easy access to insufficient material such as satellite images that are sort of incomplete maps. These have been called here no-maps.

For Navigators to use maps they must have:

1. geographic names (or forget about communicating the terrain with locals, over radio with other 4x4s, or even the simple writing about the safari to your friends, etc.)

2. landforms shapes (miss the only landmark in a 1000 kilometer might be fatal)

3. grid of 'understandable' coordinates. Digital maps should be 'geo-referenced' but that's useless if you are not good with (projections, etc.). If you don't have this, then forget about positional fixes.

4. scale bars (or no estimation of distances would be possible)

If you're into cartography (map-making) then you might like to add all those yourself to a satellite images (with all the geometrical problems that might degrade your positional fix terribly).

No-maps are some of the elements from which maps are made and together they're called in science geographic data (or geo-data for short).

Here are some major sources of such extremely helpful geographic data for our Navigational needs.

 

 

NASA GeoCover Satellite Images
https://zulu.ssc.nasa.gov/mrsid/

This site will not only save you a fortune that businesses used to pay for, but they're beautifully geo-referenced that the positional error seems to be always under 100 meters.

Tiles grid and file-names for downloading 1990 GeoCover images

They have something that confuses the navigator and that's they're processed to show maximum differentiation colors. This means that the colors of the objects are not realistic. They sometimes show deserts in all colors of the spectrum. The different  colors represent different types of rocks (even if they're not the same to your eyes in the field). Although this is confusing for the beginner, it could be extremely useful when you know how to handle it. There are ways to resolve that but they're out of our scope for here.

Unusual colors of the Sahara.
Above: elGilf elKebir (brown patch mid-upper part) and Mt Ouenat and Arkenu (black spots divided by white line  at centre of left half)
Below: in Tibesti plateau desert (Chad and Libyan borders) showing a volcano at lower left and unusual red splotches in dark green area on the right half. Quite a feast for the eye but needs work before good for navigation

GeoCover Images Dates

There are two dates downloadable from NASA as GeoCover: 1990 and 2000. For our purposes in Sahara, there are very little apparent differences in the desert between the two dates. The true difference is in the sizes of images files. While typically 1990 dates are in average 50MB, those of 2000 are in more than 200MB compressed!

Images Resolution

Beside this too much size, there is a good change in its resolution which is said to have increased from 30 meters in GeoCover 1990 to 15 meters in 2000. The Resolution number is the how much the side of a pixel measures on the real ground. Unfortunately, this increase of resolution used here is a pan-sharpening and is considered to be not a true resolution.

Generally, it's recommended to use the 2000 unless you have very good reasons for it.

[Your feedback/questions are appreciated.]

 

 

GNS
http://earth-info.nga.mil/gns/html/

 

This is the most famous Gazetteer database for Navigators on the web provided for the entire world by the American Military.

A waypoint is a point on the way to somewhere.

In this sense many of the Gazetteer's points are NOT waypoints.

Some are actually points that are meant to represent an area. The area of the desert of Selima is huge at 100,000 km² and the point with the name 'Selima Sandsheet' in the Gazetteer is not meant to be a "waypoint" at all but just a point to hold the 'geographic name' in a random point near the centre of the area.

Examples of Gazetteer points:

• a long winding wadi that stretches for--say--20 kms would have only one point near its middle with its name in the Gazetteer.
• quarries or Sabkha (quicksand)

From the above, it's obvious how Gazetteers are no-maps but could be very useful if you're into the map-making hobby.

So What's A Gazetteer Anyways?

The Gazetteer is the list of such points of 'geographic names' and their longitudes and latitudes. In Arabic it's called Mo3gam Joghraaphy ( مُعجَم جغرافى or geographic dictionary) and used to be a middle-eastern specialty since ancient times where it either had been compiled in a specialized book in Alexandria, or Babylon. It later became an index near the end of books to define locations mentioned in the text.

GNS Accuracy

The Gazetteer seems to have been degraded intentionally in some areas (seems the copyrights issue again). So you cannot use it immediately in any area until you verify it yourself against a map you trust with a scale good for your needs.

The degradation is done by resetting the seconds or minutes to zeros. It's done in certain places and not over the entire region or the gazetteer file you're using. If you like, you could check that visually and the points will seem regularly spaced like on a checkerboard.

Arabic in Digital Gazetteer

While Arabic speakers on the Net have found an easy way to transliterate some of their phonemes to Net-Latin, GNS is still using the 'formal' way. It's not very easily readable for native speakers. A better way --that might have benefited database searches too --could have been to use yet another database column for Arabic (native) letters, but for such a global database, it is understandable that this may be a an unnecessary nuisance.

Arabic Geography and Gazetteer

There's no standard translation so far between Arabic and English geographic names. Some of the Arabic words have been Latinized like 'wadi' but that has changed a bit in meaning to suit scientific needs so now in English there's a difference between wadi and valley. Many other Arabic landforms are not yet recognized in English.

 

Digital Map-Making

Incidentally, all the no-maps you'll probably come across are digital. It's beautifully suitable for map-making, but that's not for beginner Navigators.

It requires sometimes difficult GIS processing to adjust for navigation purposes. Also a professional plotting facility to suit field use (for shops in Cairo, see Maps Services).

 

[Your feedback is appreciated.]

 

Wait for more! (email for suggestions. thanks!)