|MAPC2MAPC Help pages : Digital Map Glossary
|In the section below is a list of terms and (my understanding of) their meaning....
There are two basic types of Digital Maps : Raster Maps and Vector Maps. Raster Maps are based on images or pictures. The map is made up of thousands or millions of picture elements - green for forests, blue for rivers etc. A Raster Map is clearest when it is displayed with a scale where map pixels match screen pixels. A Vector Map is based on definitions of lines, points and areas. It can be displayed clearly at many different scales but curves are often simplified to a series of straight line pieces and the result may not look as good. A vector map can be converted to a raster map but conversion the other way requires special software which does not always work well.
But my speciality is Raster Maps so for Vector definitions, you need to look elsewhere.
depth : The number of bits used to desribe each pixel. For full colour
images this is usually 24 (8bits each Red, Green, Blue). Palettised
images are usually 8bits deep.
BMP : A file containing an image. It can be black and white, greyscale, palettised colour or full colour. BMP files are not compressed so are often very big in comparison with other formats. BMP files cannot have transparent areas.
Calibration : The information that links the contents of a map to the places on the Earth that it represents. Most commonally, it is a list of pixel positions in the image and their corresponding Latitude and Longitude - bit there are other ways. Often held in a separate Calibration File but can be embedded into a file with the image.
Compression : A pixel on a map is typically represented by three fields, each of one byte, containing the intensity of Red, Blue and Green. So an image 100 pixels square is 30000 bytes of data. Compression is a way of reducing the size needed to store the image. Compression my be Lossless (the image can be exactly reconstituted) or Lossy where some information is lost.
Datum : This is the mathematical model of the world on which the map is based. The international standard is known as WGS84 but this is a result of accurate satellite-based measurement. Before satellites, measurement was difficult and the results not accurate. Each country often used its own measurements. In the UK, for example, the Airy 1830 Datums is used. A large set of datums is distributed with MAPC2MAPC and the program will convert as needed. A datum contains five pieces if information : The radius of the Earth at the Equator, how much it is flattened from a sphere and positional adjustmenst X,Y and Z
Freeimage : A set of programming tools to manipulate images. More details.
Geocaching : A pastime/game/activity that invloves searching for hidden 'treasure' with the help of a GPS unit.
GPS : Global Positioning System. A series of satellites which broadcast a signal that can be used to calculate a position. Although extremely accurate (centimetres) it is deliberately limited by the US government so that reatil units are only accurate to a few metres. GPS units are small computers that can decode the signal, pass it to another computer and/or display the coordinates and/or display a position on a map.
Grid : Grid is the coordinate system used on the map. This interacts with the projection : a map of California, for example may have an Albers projection but the grid defines the base points for the projection, the measurement units (usually metres) and the point from which the units are measured.
JNX : A Garmin file format that can contain verly large images at up to five levels of detail. Originally it was used to distribute their Birdseye imagery but the format was decoded and there are several tools that create htese files. To display them on a Garmin unit, however, an unauthorised alteration (patch) to the unit's firmware must be made. More
JPG or JPEG : A file containing an image. The JPEG method compresses images using algorithms that allow pictures to be stored with much less information but still look good. But some information is lost and for maps, especially when compressed a lot, this format can introduce blemishes in the image such as haloes around points. JPG files cannot have transparent areas
KML/KMZ : Keyhole Markup Language used by Google Earth to describe tracks, routes and maps. The language is also used to describe Garmin Custom Maps. A KMZ is a KML file that has been compressed with the Zip program.
Palette : Like an artist's palette, this means the image is created from a limited set (typically 64 or 256) colours. For most maps, there is no loss of visual quality but considerable savings in space. Palettised images my also be described as having a bit depth of 8.
Pixel : Picture Element - the smallest piece of data (a dot) in a map or on a screen..
PNG : A file containing an image. It can be black and white, greyscale, palettised colour or full colour. Any compression is lossless.
Projection : Projection is the mathematical rules used to display a representation of the curved surface of the earth on a flat screen (or piece of paper). Different projections have different advantages and disadvantages. Mercator, for example preserves directions but not distances; Plate Carree is simple but only accurate for small areas. The program understands the following : Plate Carree (Latitude Longitude), Mercator, Universal Transverse Mercator, Transverse Mercator (including British, Irish and Swiss), Lambert Conformal Conic and Albers Equal area.
Route : A list of points(latitude+longitude) that describe a planned journey. The points can be known as waypoints. In a compputer file they mayc ontain other information such as directions or links to photographs.
TIF (or TIFF) : Tagged Image file format. A file containing an image. It can be black and white, greyscale, palettised colour or full colour. TIF files can also contain calibration information whne they are called GEOTIFF. Any compression is lossless.
Tracks : A series of points (latitude+longitude) the describe a journey that has been undertaken. Often also includes altitude, date and time.
World File : File types of a World File are TFW,PGW nad JGW. A World file is a form of calibation file. It defines the coordinates of the top left of the map and the rate of change (e.g degrees per pixel) as one moves across the map.