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.
UTM Coordinates Calculations
As mentioned above, in the UTM coordinate system, meters are used and there are no negative numerical values. To explain in more detail how to calculate the x and y coordinates of the UTM system, consider the illustration of a UTM zone below.
For example, let’s take the example of zone 48 which covers the western part of West Java, Lampung, Bengkulu, parts of South Sumatra and Jambi.
One zone has a width of 6° longitude (longitude west – east), take for example zone 48 which stretches from coordinates 102° – 108° east longitude, where the central meridian of this zone means 105° east longitude. In UTM this line value of 105° east will be converted to 500,000 meters (X = 500.00 m).
As for the value of the X coordinate of a point, it depends on its position whether it is in the west or east of the central meridian zone. For example, if a point is to the west of the central meridian, then the UTM coordinate value is 500,000 m – the distance of the point is perpendicular to the central meridian (in meters).
On the other hand, for the value of latitude or Y UTM coordinates, first the equator with a value of 0° is converted to a value of 10,000,000 meters specifically for areas south of the equator (south latitude region in Geographical coordinates). As for the area north of the equator (north latitude in Geographical coordinates) then the value of the equator is 0 meters. The value of the Y coordinate in the area south of the equator is 10,000,000 – the distance of the point is perpendicular to the equator (equator), while for points in the area north of the equator, the Y coordinate value is the distance of the point perpendicular to the equator (equator).
UTM Coordinate Writing Rules
Because it is a western product, the standard writing or notation of UTM coordinates follows the English language. The writing rules follow the rules of Z-E-N (Zone – Easting – Northing). The first written zone is horizontal zone (without spaces) vertical zone (space) x coordinate value (without spaces) mE (space) y coordinate value (without spaces) mN / mS. For the record there is no division of west and east in the x-coordinate, there is only a division for the y-coordinate that is north or south of the equator. An example of writing standard UTM coordinates is 48M 700618.99 mE 9268568 mS
Example of UTM coordinates calculation
Take the example of the Ampera Bridge point in Palembang City based on Google Map as follows, the value of the longitude (X) = 104.7535215° (BT) and latitude (Y) = -2.9873515° (LS), this means that the position is located to the west of or before the Central Meridian zone 48 (105° east) and to the south of the equator. Then according to some literature the distance of 1 ° at the equator is approximately 111,319.9 meters.
To find the value of the X UTM coordinates, the distance calculation is 105° – 104.7535215° = 0.2464785°, which means that the distance from the Ampera Bridge point to the Central Meridian zone 48 (105° east longitude) is 0.2464785 x 111.319.9 m = 27,437,96197 m. This means that the X UTM coordinates of the Ampera Bridge are 500,000 – 27,437,96197 = 472,562,038 m.
Meanwhile, to find the value of the Y UTM coordinates, the calculation of the distance is 2,9873515 x 111,319.9 m = 332,551,6702 m. Because it is south of the equator, the Y UTM coordinate value is 10,000,000 – 332,551,6702 = 966,7448.33 m.
Thus the complete UTM coordinates of the Ampera Bridge are Zone 48M, X = 472.562.038 m, Y = 966.7448.33 m.
Some Matters or Issues related to UTM Coordinates
- In GIS or Remote Sensing software, it is rarely used to write standard UTM zones with letters such as 48M, the standard usage is usually only on GPS devices. The writing that is often used is the division of the UTM zone according to its latitude, whether it is north or south of the equator (N=North/North, and S=South/South), so the writing is for example a 48S or 48N zone.
- The UTM coordinate system in GIS is usually chosen when calculating distances, areas, or volumes.
- In some large areas more than one UTM zone is covered, including from latitude (into South-North), because the provisions must determine which UTM zone to enter, usually the choice of consideration taken is the widest UTM zone coverage in the area. As a consequence, areas that are not actually included in the selected UTM zone will have incorrect coordinate values and experience distortion. In a case like this, it is better if the layout specifically uses Geographic or Lat-Long coordinates. Another alternative is to use a similar projection system but which is not zoned, such as the World Cylindrical Equal Area in particular to calculate the area, but the results will be different from UTM.
- The spheroid or globe that is commonly used in UTM is usually WGS 1984, but some use other spheroids.
- To determine what longitude coordinates in the territory of Indonesia are in the UTM zone, the quick formula that can be used is 30 + (longitude coordinates/6°). The number after the comma in the result (regardless of magnitude) is rounded to 1. For example, 118° means 118°/6° = 19,667 or rounded up to 20, so 30 + 20 = 50, meaning 118° is in the UTM 50 zone.
- If you need a UTM zone division in shp format (for the world and specifically for Indonesia) that you can use to overlay with your map, please download it here.
Reference
Summarized from various sources, among them are;