Overlay Mapper Foregrounds
Layer Notes - Grid Locators
The 'Grid Loc' foreground overlay shows the IARU Grid Locator system, used to identify positions by Radio Amateurs on HF and VHF, and in some contests and operating awards. First proposed at a conference in Maidenhead, England in 1980 (and hence sometimes called Maidenhead Locators), grid squares allow three levels of precision (two, four or six character) to define a station's latitude and longitude. The world is divided into a grid of squares that are ten degrees of latitude high by twenty degrees of longitude wide. Each square is identified by a different two-letter code. For example, the District of Columbia is in location 'FM'.
Each of these squares is further divided into 10 on each axis, giving 100 sub-squares, each of which is one degree of latitude high by two degrees of longitude wide. Each of these squares is identified by the two letters of the alphabet (generally shown in capitals), and two numeric characters. For example, Washington,DC is in location FM18.
For higher precision, each grid square may be further divided into 24 on each axis, giving 576 sub-squares, each of which is 2.5 minutes of latitude high by 5 minutes of longitude wide. Each of these sub-squares is identified by the two letters of the alphabet and two numeric characters as before, plus two lower-case alphabetical characters. Using this system, a station's location is thus identified to within better than 5.6 nautical miles anywhere on the surface of the Earth. For example, the center of Washington, at latitude 38.92N and longitude 77.01W, is in location FM18lw.
The easiest way to convert a position to a Grid Locator is to move the cursor over any map. The latitude/longitude and your grid-locator will be shown in the Status Bar, at the bottom of your browser. For more information, visit the Status Bar help-page. To convert a Grid Locator to a position, you can enter it on the 'Search > Position' tab-page.
Layer Notes - Latitude/Longitude Grid
The 'LatLon' foreground overlay allows us to find the location of any point on the currently selected map. The colors chosen were designed to be visible against a range of backgrounds, without obscuring more important detail. Numbers are included to identify the lines. Those identifying lines of Longitude (North-South) are of three digits, whilst those identifying lines of Latitude (East-West) are of two digits. For more exact positional information, the latitude/longitude of your current cursor-position is shown in the Status Bar, at the bottom of your browser. For more information, visit the Status Bar help-page.
A bit of history: Although the first map reference system was a simple rectangular grid, developed by the Chinese, later map-makers realised that they had to take account of the earth as a sphere. The Greeks derived from the Babylonians (present-day Iraq) the idea of dividing a circle into 360 degrees, and the Greek geographer Eratosthenes (c.276-194 B.C.) was the first to calculate the circumference of the earth, and was reported to have made a world map based on the concept of the earth's sphericity. From this, the Greeks went on to develop the system of spherical coordinates still in use today.
The poles at each end of the earth's axis provide the reference points in its rotation in relation to the celestial sphere. Parallel circles around the earth are degrees of Latitude, and express the idea of distance North or South of the Equator. Lines of Longitude running North or South through the poles express East-West distances. One Meridian is chosen as the meridian of origin, known as the Prime Meridian. In 1884, at an international conference in Washington DC, most countries agreed to adopt the Prime Meridian through the Royal Greenwich Observatory in England, and to calculate Longitude to 180 degrees East and West of Greenwich.
Layer Notes - Country and Regional Names
Throughout the Global Overlay Mapping System, country names are generally shown in upper-case (i.e. all capital letters) and a 10 point font, whilst region names are shown in mixed-case, and an 8 point font. However, some confusion occurs where what is classed as a country name for Amateur Radio purposes (i.e. separate DXCC status) may not be a country name in the 'real world'. Examples of this include Wales and Scotland. To cope with this confusion, the standard adopted here has been to give such 'countries' a separate color on the 'Country' overlay, but show their names in mixed-case, region-style on the 'Names' overlay. Some discrepancies may be noted to this rule - this is generally due to a lack of space.
Some Russian Oblasts (region names) have had the suffix 'skaya; omitted due to lack of space. Other names may be truncated for the same reason.
Layer Notes - Capital and Other Cities
The positions of all towns and cities on the map overlays were entered by hand, and were calculated using custom software developed specifically for the Global Overlay Mapping System. It is believed to be correct to ±1 pixel, though any errors in the original data would of course be reflected in the final output. The city data, converted from d:m:s to decimal, has been compiled into a number of plain text, comma delineated, continental lists which are available for sale separately. For further information, please contact the author.
Choice of cities. Although all capital cities, and most state capital cities were chosen to appear on the maps, the design-brief was to give a good idea of national coverage, whilst trying to avoid clutter and over-crowding of the maps. Therefore, certain large cities have been omitted if very close to more major ones in crowded areas, whilst in less populated regions, smaller cities have been chosen to indicate local population centres.
Layer Notes - Amateur Radio Prefixes
Prefixes are shown in a number of ways - the design-brief was to make them as quickly understandable as possible. Prefixes are split into three main groups, which are shown on the map as follows:
One prefix covers the whole country, without any divisions. Typical examples are C6: The Bahamas. The prefix overlay shows the prefix itself, with no other additions.
A country has a number of prefix areas, each of which correspond to individual country-regions. A typical example of this is VE:Canada. The prefix overlay shows the prefix itself, plus the region-borders that correspond to each prefix sub-division.
A country has a number of prefix areas, each of which correspond to regions composed of a number of individual country-regions. A typical example of this is W:United States of America. The prefix overlay shows the prefix itself, plus a red border that shows how a number of individual country-regions are grouped to form each prefix sub-division. The region-borders themselves have not been included, but can be added by turning on the 'State' foreground or background overlay.
The size of prefixes used throughout the Global Overlay Mapper is based purely on map-room and clarity of information. No other meaning is intended or should be inferred.
Layer Notes - IOTA (Island On The Air)
This foreground overlay shows the IOTA reference numbers for islands qualifying in the popular 'Islands On The Air' award-scheme, also used in the annual IOTA competition. Created in 1964 by Geoff Watts, a leading SWL and editor of the famous DX News Sheet (DXNS), it was taken over by the Radio Society of Great Britain (RSGB) in 1985, and is an amateur radio activity program designed to encourage contacts with island Amateur Radio stations world-wide. The ocean's islands have been grouped into some 1200 IOTA groups, full details of which can be found in the highly recommended 'RSGB IOTA Directory', published by and available from the RSGB's main website. A list of all current IOTA island groups, plus the latest IOTA news, can be found at the RSGB's IOTA website. There are also many internet sites which provide additional information, lists, and photos of island expeditions.
The positions of each IOTA label on the GOM overlay correspond to the calculated center of each IOTA entity area. IOTA areas are mostly defined as being contained within a rectangular area of specified Latitude and Longitude range. A full sortable/searchable list of all North American IOTA entities has been provided; you can access this by clicking 'Tools' > 'Table Viewer' > 'NA IOTA'. This data has been provided here at no charge for non-commercial home use only, and remains the copyright of the RSGB.
You can jump straight to any North American IOTA number without having to know its position by using the 'Select Map by Name' > 'IOTA Reference Numbers' map-select tool, on the Map Navigation page.
A useful and unique IOTA resource is located at the EI8IC website. This page lets you enter your own 6-digit Grid Locator; the page will then return a list of all IOTA entities sorted by continent with distance and bearing to each from your own QTH.
Layer Notes - Flags
The 'Flags' foreground overlay contain national, state, and province flags, everyone of which has been hand-edited and resized for this application. The aspect ratio of each flag has been adjusted to a constant value for clarity.
Important: As is stated in the GOM License, you are not allowed to copy the flags presented here, or use their images in any other application. If you wish to obtain a complete set of flags that may be used commercially or for most other purposes, please visit the EI8IC website, where you can purchase the full set for only 10 US Dollars online with credit-card or PayPal account.