Understanding Map Scales, Contours, UTM Coordinates, Orthophotos, and GPS
Understanding Map Scales and Contours
Scale refers to the relationship between distances on a map and the corresponding distances on the actual terrain. Contours are defined as imaginary lines on a map that join points of equal elevation. The height difference between each contour line is called equidistance and varies depending on the map’s scale.
Key Properties of Contour Lines
Contour lines have several properties that are important to consider when interpreting a map:
- Every contour line closes on itself, although sometimes it may extend beyond the map’s boundaries.
- Contour lines never intersect each other.
- A contour line can never split into two or more lines.
- Two or more contour lines can merge only on vertical slopes (90°).
- The area between two contour lines is assumed to have a uniform slope.
Distances and Slopes
On a map, we can identify different types of distances:
- Reduced Distance (AC): The horizontal distance between two points on a map.
- Geometric Distance (AB): The straight-line distance between two points, taking into account the elevation difference.
- Natural Distance: The actual distance along the ground between two points, following the terrain’s undulations.
The relationship between reduced distance, geometric distance, and slope is determined by the angle (α) and can be calculated using the following formulas:
- Geometric Distance = Reduced Distance / cos(α)
- Slope = Geometric Distance * sin(α)
UTM Coordinates
The Universal Transverse Mercator (UTM) system divides the Earth into 60 zones, each spanning 6° of longitude. Each zone uses a cylindrical projection tangent to its central meridian. The origin is the antimeridian of Greenwich. Each zone is identified by a number, starting with zone 0 at the 180° meridian.
Each zone is further divided into areas by parallels every 4° latitude, starting at 80°30′ South. These areas are identified by capital letters from C to X, excluding I, LL, N, and O. For example, Catalonia is located in zone 31T.
Orthophotos
Orthophoto mapping is based on vertical aerial photographs or satellite images. These images can be in visible light or infrared and are geometrically rectified to a uniform scale. Various scales are available, from satellite imagery of all of Catalonia at 1:500,000 to complete sets at 1:5,000 scale. Orthophotos allow us to appreciate the real depth of an area, but they lack the elevation information provided by contour lines.
The GPS System
The Global Positioning System (GPS) consists of a network of 24 satellites called NAVSTAR, orbiting approximately 20,200 km above Earth, and GPS receivers. These receivers determine our position anywhere in the world, day or night, and under any weather conditions. The satellite network is owned by the US Government.
Each satellite transmits two types of data: ephemeris data, which corresponds to its exact position and time in UTM (Universal Time Coordinated), and almanac data, which provides information about the other satellites in the network and their orbits. This data is continuously transmitted via radio signals. When we turn on our portable GPS receiver and point the antenna towards the sky, it begins to capture and receive these satellite signals.