Not infrequently, people express surprise that we make maps: “But aren’t there maps available for most of the world by now?” or other questions along this line.

It is no surprise to members of IMTA that the whole world is not yet mapped to the precision of the United Kingdom, the United States, France or Germany, for example-even acknowledging the deficiencies of some of that mapping. But, of course, those of us involved in the mapping business also recognise the vast array of specialist maps which are now produced in both paper and digital form. These range from cadastral maps, thematic maps, demographic maps and so forth, all addressing the need for spatial information in a huge variety of contexts and formats.

Quite recently, we had a requirement for a rather unusual world map which posed its own unique blend of production problems. We were asked to produce a world map of ground (electrical) conductivity incorporating a highly accurate coastline-the best possible.

This probably requires an explanation. Up until around 1980, high frequency (HF) radio links were the principal means of communicating over medium-to-long distances. In practice, there was a major subdivision in the HF band: long-haul communications that passed through the ionosphere (often called sky-wave), and medium-to-short haul communications that relied on ground waves.

All this changed with the advent of satellite communications and, from the late 1970s, HF communications was superceded for both long- and short-haul communications. This transition was evident in all walks of life-from military systems and diplomatic radio, to maritime communications and even domestic radio in hitherto poorly served areas such as Africa and elsewhere.

As with so many innovations, the pendulum has swung back again and HF communications are once more in vogue, due largely to the over-crowding of satellite channels and, as far as the military are concerned, to the vulnerability of satellites to attack and/or interception.

Changing Times
In the same period, HF communications have moved on, with the frequency band expanding from 2-30 MHZ to 1-40 MHz or even more. In addition, the technical implementation of HF radio systems has changed beyond recognition. In particular, modern military radio systems use frequency hopping and spread-spectrum technologies to maintain secrecy as well as the sophisticated encryption procedures which are now commonplace.

Frequency hopping is based on the precept that the radio link does not operate on a single frequency; rather, that the frequency of transmission is changed (often rapidly) to confuse the enemy. Naturally, there are technologies in place to ensure that the transmitter and the receiver move around the frequency spectrum in synchronism, albeit in an apparently random manner.

Although this approach has been used for a number of years, there is an extra parameter which is beginning to be exploited in the ever-sophisticated game of secure radio transmission. The highest frequency at which HF communications can take place over a given route is dependent on a number of factors, amongst which is the ground conductivity of the path. Put another way, the path length over which propagation can take place is frequency-dependent, and the “cut off” above this frequency is quite sharp. Although the mechanism is different, any user of a mobile telephone will be familiar with this effect: It is all too easy to move out of the range of a transmitter quickly - sometimes in a remarkably short distance - and this is critically dependent on both the frequency and the environment surrounding the propagation path.

Thus, to ensure that radio transmissions are as secure as is reasonably possible, it is now practical to operate at a frequency, and at a level of transmitter power, that ensures that the signal only just reaches friendly forces and does not propagate onward to hostile forces. This is of even greater importance for clandestine operations.

The Relevance of Mapping
As stated above, among the parameters which affect this propagation path - in particular, the length of the path - is ground conductivity, i.e., the environment over which the radio signal passes.

The conductivity of water features is well known, including such aberrations as the high conductivity of the Dead Sea (the conductivity of sea water is generally in the range 4,000 to 5,000 mS/m but is ~27,000 for the Dead Sea!). By comparison, dry ground is in the range 1-to-10 mS/m, i.e., a ratio of 1,000:1 between land and sea.

This huge ratio is of major tactical importance to military authorities contemplating seaborne landings where a mixed transmission path is involved, e.g., including a seawater (or high conductivity) component, and a land (low conductivity) component, especially if landings are contemplated in an arid region.

With the universal use of GPS in such operations, the location of both transmitter and receiver are known to high levels of accuracy. Taking into account the large ratio between propagation efficiency over land and sea means that it is vital to know the demarcation between high and low conductivity areas to a high degree of accuracy so as to be able to predict the overall transmission characteristics. In turn, this permits operators to choose the optimum frequency to assure good communications, yet deprive the enemy of adequate levels of signal for the purposes of interception.

This rather complex scenario relies on sophisticated radio propagation modelling software, in conjunction with comprehensive conductivity maps plus an accurate coastline around all major oceans and seas, especially those which may be of military interest. All of which we were delighted to produce!

In essence, for tactical operations in a potentially hostile environment, it is important to know the relative position of the coastline between transmitter and receiver to an accuracy commensurate with the 1,000:1 ratio of ground conductivity in the two regions.

Summary
In response to the opening question (But aren’t there maps available for most of the world by now?), the answer perhaps should be “Yes, but not in a form which is increasingly required by modern applications of spatial data.”

Indeed, there may be excellent conductivity maps, and there may be highly accurate maps of coastlines on a country-by-country (or regional) basis, but all too frequently, such maps (or such datasets) aren’t combined in a manner required for novel applications. For this reason, if for none other, we believe that the role of the specialist (IMTA) map producer will be sustained for many years to come.

For further information, please contact Geo Strategies at:
	Geo Strategies Ltd			Geo Strategies SA
	St John's Innovation Centre			Str G-ral V. Milea, 10A
	Cowley Road			Sibiu 550331
	Cambridge  CB4 0WS			Romania
	United Kingdom
	Tel:	+44 (0)1223 205080		Tel:	+40 (0)269 210832
	Fax:	+44 (0)1223 205081		Fax:	+40 (0)269 211165
	Email:	maps@geo-strategies.com		Email:	maps@geo.strategies.ro