Mapping and GNSS basics

A Global Positioning System, GNSS (commonly termed GPS) is a set of satellites that transmit data to permit a GNSS receiver to determine its geographical position.  Currently there are three international operating systems – GPS (USA), GLONASS and Galileo.  Provided a GNSS receiver has a clear view of the sky it is effective anywhere on the earth's surface at any time.

Handheld GNSS receivers have the facilities to manage waypoints and tracks.  Some GNSS receivers have the ability to display maps.

Waypoint:   A waypoint (also known as point of interest, POI) is a recorded location.  A waypoint can be recorded in a GNSS receiver by

• marking the current location,
• keying in a location reference or
• downloading a location reference from a computer.

The GNSS receiver can be used to find the way to a waypoint.  A waypoint can be uploaded to a computer.

Track:     A track is a recorded path.  A track can be recorded in a GNSS receiver by

• moving with the GNSS receiver active or
• downloading the track from a computer.

The GNSS receiver can be used to guide movement along a track.  A track can be uploaded to a computer.

A location in Sydney	A location in Dublin	Format
–33.8700°, 151.2183°	53.3345°, –6.2508°	D.dddd
–33deg; 52.20', 151° 13.10'	53° 20.07' N, 6° 15.05' W	D M.mm
33° 52' 12" S, 151° 13' 6" E	53° 20' 4", –° 15' 3"	D M S.s
Some examples of latitude, longitude formats

A geographic location is specified by a reference within a coordinate system.  There are three reference systems commonly used in the field.

Latitude and Longitude or geographical coordinates:

The geographical coordinate system is useful on small and medium scale maps;it is not as convenient as the UTM system on large scale maps.  The parallels of latitude run east-west, the meridians of longitude run true north-south from pole to pole.  A location is given by the latitude north or south of the equator and the longitude east or west of the zero meridian.  The units are generally degrees, minutes and seconds though there are format variations.  There are 60 seconds in a minute and 60 minutes in a degree.  A negative sign on latitude can be used to indicate south, a negative sign on longitude can be used to indicate west.  A degree of latitude or a degree of longitude at the equator is approximately 110 kilometres; a minute of latitude or a minute of longitude at the equator is approximately 1.9 kilometres – a nautical mile.

UTM/UPS:

The Universal Transverse Mercator/Universal Polar Stereographic, UTM/UPS, or commonly just UTM, projection system is convenient for medium or large scale maps.  The world is divided into 60 longitudinal zones (notated by a zone number, 0 – 59) by meridians spaced at 6 degree intervals (0°, 6°, 12°, ...).  On each of these zones a square grid is projected.  In addition there are 20 lateral zones (notated by an alphabetical zone characters).  A location reference on this grid is specified by a zone number, a zone character, an easting (6 digits) and a northing (7 digits, though just north of the equator the northing may have leading zeros).

Distribution Mapping Software has a world map of UTM zones.

The UTM system can be useful on medium and large scale maps because the coordinates are metre based and a grid overlay is square and regular – commonly 1 kilometre.  If a map is marked with a 1 kilometre grid it may be the practice in a radio net to give local locations as a 6 digit grid reference.  The appropriate numerals are underlined in the diagram to the right – 830 708.

MGRS:

The Military Grid Reference System, MGRS, is a variant of the UTM system and uses alphabetical identifiers to reduce the likelihood of errors in manual data transfer, digital technology has reduced the use of this system.  The alphabetical identifiers define a 100 km square.

Coordinate Conversion:

GPSVisualizer provides a versatile online coordinate converter.

A possible trap for the unwary:

Both the UTM system and the MGRS system use the alphabetic characters C through to X excluding I and O but including N and S as lateral zone indicators.  However in some UTM system applications the symbols N and S serve as hemisphere designators as an alternative for zone indicators.  A general rule is – do not substitute N and S for zone indicators unless explicitly required.

The zero meridian passes through a brass marker at Greenwich (strictly, in the digital age the zero meridian is approximately 100 m east of the brass marker) – the marker is the reference for the traditional geographic coordinate system.  In an analogous (but simplified) fashion there is a reference – a datum – for global positioning systems – in fact for historical and other reasons there are many references.  Fortunately the World Geodetic System 1984, WGS84, is dominant internationally.  Wherever a datum choice is presented choose WGS84.

In the case of a hard copy map, the scale, when expressed as a ratio, "1:100,000" for example, is unambiguous – 1 cm on the map represents 1cm X 100,000 = 1·000 km on the ground.  In the case of an electronic map image, the situation is not as straightforward.  On a laptop, a map image file with a ground extent of 25 km may be displayed as 250 mm wide (a scale of 1 : 100,000), while the same file presented on a video projector screen may be ten times the size (a scale of 1 : 10,000).  Similarly a map file printed on an A4 sheet will be at a different scale to the same file printed on an A3 sheet.  A scale bar, however, remains valid at any degree of enlargement.

With an application such as OziExplorer that permits the map image to be zoomed, the concept of physical scale is depreciated.  In practice though, the user is concerned with the extent of the map image – does it cover the area of interest? – and the scale of the map image – can it reasonably record the detail required?  There is a trade-off between extent, scale and map image file size.  To aid evaluation of the suitability of a map image in a particular application MapToGround provides two additional parameters for each map image.

1) Map Size:     The size of the land area (width by height) covered by the map.

2) Scale indicator:     The scale indicator, for example, ' 1.0 km ', is representative of the map at '100%', 'actual pixels' or 'natural size'.  Regardless of the web page zoom level or pixel pitch the scale indicator will be valid for the neighbouring map extract.  If the MapToGround web page is displayed without zoom, the scale indicator will be valid if the map image is displayed (in OziExplorer say) at '100%', 'actual pixels' or 'natural size'.

Large scale, small scale:

The terms, large scale, small scale, are relative.  A map may be considered large scale in one context (logistics planning, say), yet small scale in another context (camp design, say).  The terms large, small, are not intuitive – 1:25,000 is a larger scale than 1:100,000.  A possible memory jogger is that items appear larger on a large scale map and smaller on a small scale map.  Consider a map of a country and a map of a town within that country.  On the map of the town – a large scale map – buildings may be large enough to be distinguishable.  On the map of the country, the buildings are too small to be visible.