Acquiring photography of a study area, with either administrative or physical boundaries sometimes requires thousands of photographs or spatial data spread along many paths or rows, depending upon the size of the study area and the spatial scale.

Map Scale: Map scale is the relationship that exists between distance on a map and the corresponding distance on the earth. It is usually expressed in the following form: 1:50,000, meaning that 1 unit of measurement on the topographical sheet represents 50,000 of the same units on the earth's surface. The scale ratio 1:100,000 means that one unit of distance on the map represents 100,000 of the same units of distance on the earth. So, on a 1:50,000 scale map, one cm on the map equals half a kilometre on the ground because half kilometre has 50,000 cm. Because the scale ratio is a constant, it is true for whatever units in which the fraction is expressed.

A 'large' scale map is one in which a given part of the earth is represented by a large area on the map. Large scale maps generally show more detail than small scale maps because at a large scale there is more space on the map to show the features. Large scale maps are typically used to show site plans, local areas, neighbourhoods, towns etc. 1:5,000 is an example of a large scale. A 'small' scale map is one in which a given part of the earth is represented by a small area on the map. Small scale maps generally show less detail than large scale maps, but covers large parts of the earth. Maps with regional, national, and international extents typically have small scales, such as 1:1,000,000. When comparing two scales, it is important to remember that the larger the number in the scale expressions, the smaller the scale. Large scale maps typically show more detail than small scale maps, whereas on smaller scale maps there is simply not enough room to show all the available detail, so features such as streams and roads often have to be represented as single lines, and area features like cities, have to be shown as points. Table 1 is a useful conversion chart.

Geographic Coordinate System: One of the most common coordinate systems in use is the geographic coordinate system, which uses degrees of latitude and longitude to describe a location on the earth’s surface. Lines of latitude have the reference latitude as the equator which at 0 degree divides the earth into northern and southern hemisphere. All latitudes run parallel to the equator and divide the earth into 180 equal portions from north to south (or south to north) and each hemisphere is divided into ninety equal portions, each representing one degree of latitude. In the northern hemisphere, degrees of latitude are measured from zero at the equator to ninety at the north pole. In the southern hemisphere, degrees of latitude are measured from zero at the equator to ninety degrees at the south pole. To simplify the digitisation of maps, degrees of latitude in the southern hemisphere are often assigned negative values (0 to -90°). Wherever you are on the earth’s surface, the distance between lines of latitude is the same (60 nautical miles), so they conform to the uniform grid criterion assigned to a useful grid system. Lines of longitude, on the other hand, do not stand up so well to the standard of uniformity. Lines of longitude run perpendicular to the equator and converge at the poles. The reference line of longitude (the prime meridian) runs from the north pole to the south pole through Greenwich, England. Subsequent lines of longitude are measured from zero to 180 degrees east or west (values west of the prime meridian are assigned negative values for use in digital mapping applications) of the prime meridian.

At the equator and only at the equator, the distance represented by one line of longitude is equal to the distance represented by one degree of latitude. As you move towards the poles, the distance between lines of longitude becomes progressively less until, at the exact location of the pole, all 360° of longitude are represented by a single point. So, using the geographic coordinate system, we have a grid of lines dividing the earth into squares (Fig.1) that cover approximately 4,773.5 square miles at the equator, a good start, but not very useful for determining the location of anything within that square.

Decimal Degree System: An alternative method of notation in the geographic coordinate system, often used for many GIS applications is the decimal degree system. In the decimal degree system, the major (degree) units are the same, but rather than using minutes and seconds, smaller increments are represented as a percentage (decimal) of a degree. The decimals can be carried out to four places, resulting in a notation of XX.YYYY, XXX.YYY. When using four decimal places, the decimal degree system is accurate to within ± 36.5 feet (11.12 m). However, because the accuracy of the fourth decimal place is often uncertain, decimal degree coordinates are often rounded to three decimal places. This results in an accuracy of ± 364.8 feet (111.2 m). To give you an example of how the two systems of measurement compare, the location of Point `X’ expressed using minutes and seconds is 30°51’36" N, 76°25’45" E. Using decimal degree notation this same location is written as 30.8600° N, -76.4292° E. Another referencing system is universal transverse mercator coordinate system, grid on a map unlike the geographic referencing system is constant from north to south and is easy to use.

UTM - Universal Transverse Mercator Geographic Coordinate System A mercator projection is a ‘pseudo cylindrical’ conformal projection (it preserves shape). What you often see on poster-size maps of the world is an equatorial mercator projection that has relatively little distortion along the equator, but quite a bit of distortion towards the poles. What a transverse mercator projection does, in effect, is orient the ‘equator’ north-south (through the poles), thus providing a north-south oriented swath of little distortion. By changing slightly, the orientation of the cylinder onto which the map is projected, successive swaths of relatively undistorted regions can be created. This is exactly what the UTM system does. Each of these swaths is called a UTM zone and is six degrees longitude wide. The first zone begins at the International Date Line (180°, using the geographic coordinate system). The zones are numbered from west to east, so zone 2 begins at 174°W and extends to 168°W. The last zone (zone 60) begins at 174°E and extends to the International Date Line. The zones are then further subdivided into an eastern and western half by drawing a line, representing a transverse mercator projection, down the middle of the zone. This line is known as the ‘central meridian’ and is the only line within the zone that can be drawn between the poles and be perpendicular to the equator (in other words, it is the new ‘equator’ for the projection and suffers the least amount of distortion). For this reason, vertical grid lines in the UTM system are oriented parallel to the central meridian. The central meridian is also used in setting up the origin for the grid system. The origin for north-south values depends on whether you are in the northern or southern hemisphere. In the northern hemisphere, the origin is the equator and all distances north (or ‘northings’) are measured from the equator. In the southern hemisphere, the origin is the south pole and all northings are measured from there. Once again, having separate origins for the northern and southern hemispheres eliminates the need for any negative values. The average circumference of the earth is 40,030,173 meters, meaning that there are 10,007,543 meters of northing in each hemisphere.