The UTM or Universal Transverse Mercator coordinate type is one of the coordinate types that is often used in mapping and also in managing spatial data. These types of coordinates have become a standard in geographic information system applications and remote sensing. What and how exactly are UTM projections and coordinates?
This coordinate system is somewhat unique because it uses a slightly complicated calculation system. But actually if you look more closely, it is actually to make it easier for users. Among other things, UTM avoids negative numbers in its coordinates, unlike those commonly used in the Cartesian coordinate system, which is adopted in the Geographic coordinate system or Latitude Longitude (latlong).
Why is that? because the UTM coordinate system is designed so that it can represent the earth’s surface in a completely flat plane (close to real conditions) and also shows the distance between two points on the earth’s surface in near-actual conditions. The coordinate unit he uses is also the meter, and because it uses a standard distance unit, of course, negative numbers must be avoided (no negative distance).
Transverse Mercator Projection
To be able to understand a little how the UTM coordinate system works, you have to start from understanding the projection process in making maps or the earth’s surface. Here we will not explain what and how map projection is, but will go directly to a map projection known as Mercator Projection (Mercator Projection). Look at the illustration below;
In the picture, it appears that the globe is projected onto a flat plane in the form of an upright cylinder. Through this projection, it will be seen that only the area at the equator (equator) and its surroundings are truly ‘sticking’ to the projection plane. Areas that are further away from the equator (more north and south) will of course experience more distortion (shape) if they are depicted in the projection plane, because they ‘do not stick’ to the projection plane.
Even though most humans on earth do not only live in the equatorial region but are also scattered in the north and south of the equator, some even live in areas that are close to the poles or maybe even live at the poles. For this reason, the Mercator Projection was slightly modified so that it could cover a more representative area of the earth. So then made (modification) the position of the cylinder plane is not standing (upright) but made transverse or better known as the Transverse Mercator Projection, as illustrated below;
After being made a transverse cylindrical projection plane, the earth’s surface that ‘sticks’ to the projection plane is even more, from the north pole to the south pole. In this plane, the Meridian line (longitude) is the center line of the projection plane. However, this projection still contains weaknesses, because the area that is really attached to the projection plane (does not experience shape distortion when depicted on the projection plane) at one time of projection is only 6° wide, i.e. 3° to the west and east (see line green) from the meridian (see yellow line).
In this projection condition, the region inside 6° (‘sticking’) will look slightly smaller in shape with a scale factor of less than 1 (scale factor < 1) to be exact 0.9996, while the region outside it will be increasingly distorted with a scale factor > 1. The scale error in each zone is less than 0.1%.
World UTM Zones
Therefore, in the UTM system, so that the projection can cover the entire earth’s surface, it is done 60 times (360° /6° = 60) or in other words the earth’s surface area (spheroid) is divided into 60 zones. Zone 1 starts from the Anti Meridian (180 degrees west/east longitude at Geographical coordinates), then goes east every 6°, until zone 60 ends at the same place.
As for the ‘zone of latitude’, it is divided by 8° starting from the equator to the north and south with a letter code system. Zone 1 is between 180° West and 174° West, Zone 2 is between 174° West and 168° West, and Zone 60 is between longitudes 174° East and 180° (International Date Line).
How About UTM Zone of Indonesia?
As has been generally informed, that the territory of Indonesia, especially the mainland, is geographically located between the coordinates of 91° – 141° East Longitude and 6° North Latitude – 11° South Latitude. So for the UTM zone the longitude will be 30 + (91°/6°) to 30 + (141°/6°), or between zones 46 – 54 (rounded up). As for the latitude zones covered in L, M, and N. As can be seen in the illustration below.
Take for example one UTM zone in Indonesia which includes the western part of West Java, Lampung, Bengkulu, parts of South Sumatra and Jambi. The area is geographically and in multiples of 6° longitude located between 102° – 108° East Longitude. So in the UTM system the area is in zone 48. For objects in that area on a map with a UTM coordinate system it will be written ‘UTM Zone 48M’, but in GIS / Remote Sensing software it is usually written Zone 48S.