But before that, we have to remind ourselves of what is latitude and longitude.
Earth has an invisible grid, that helps us determine every little position on it by just two numbers, called coordinates.
Every location on earth has these two coordinates, which are latitude number and longitude number.
Lines of Latitude :
At the equator, we are at 0º of latitude.
Lines of latitude run parallel up to 90º North or 90º South.
Lines of latitude are apart from each other by 69 miles.
For that, to identity the exact latitude coordinate of different places within this 69 miles, each degree of latitude is divided to 60 minutes
And the same thing happens every minute.
In every minute of latitude, we have 60 seconds.
All that to add position precision to this system.
So, if we want to give someone our latitude position, we will write to him something like 32º42’54’’N.
The more we move away from the equator to one of the poles, the more degrees number of latitude go up until 90º.
Lines of longitude :
Lines of longitude are lines that help us point to how far a position is located east or west to the Greenwich line.
It’s similar to the latitude lines, they are measured using Degrees, Minutes, and Seconds but they are not parallel and they intersect at the poles.
We have 180 vertical longitude lines to the east of Greenwich and another 180 longitude lines to the west of Greenwich.
So to express the longitude of a position we always say, Xº East or Xº West.
If I add the longitude coordinate of 117º09’25’’W to the altitude coordinate expressed before, we will be talking about the coordinates of San Diego.
How to differentiate between lines of latitude and lines of longitude:
If you found it hard to differentiate between lines of latitude and lines of longitude, I have a tip for you.
If we are talking about a mountain peak and I say altitude, you directly add the term high to altitude.
Do the same thing with the altitude lines, they are always vertical from north to south (or in reverse).
This is a little tip to remember the orientation of latitude and longitude lines.
Order of the coordinates
My second tip is for the order of the coordinates. Because, if we reverse the order of the coordinates, we will be talking about a different position.
To remember that you have to start always with the latitude coordinate, think of the alphabetical order. Latitude comes first in the alphabetical order and the same thing happens in the set of coordinates.
That’s all to remind ourselves of what is the lines of latitude and longitude.
Now let’s continue with elements of the center of our map sheet.
- LATITUDE AND LONGITUDE COORDINATES
At the four margin corners of the map, we have the geographic coordinates (Latitude and Longitude) in degrees minutes seconds format.
We have additional Latitude and Longitude coordinates in just “minutes and seconds” format, and sometimes in just “minutes” to avoid having too many numbers in the map margins.
To know the exact interval used in this map, just take two coordinates and compare them. In this map, we have a coordinates interval of 2 minutes and 30 seconds.
- STATE PLANE COORDINATE SYSTEM
The second value we found on the map margin is the State Plane Coordinate System value. In this example, it’s 3 030 000 feet.
If we have the latitude and longitude grid at the international level, the State Plane Coordinate System is the same thing, but just within the united states level.
But, why we have two geographic coordinates systems if they are similar.
Good question, the difference between this and the latitude and longitude grid system is that the State Plane Coordinate System follow the boundaries of the state.
There are three projections for the State Plane Coordinate System.
Each state, according to her shape determines the projection that she will use on its maps.
The states that are wide, usually use the Lambert Conformal Conic projection, the other ones that are narrow, use the Transverse Mercator Projection.
Like the projections, each state chooses whether its units will be measured in feet or meters.
That’s all for the State Plane Coordinate System, it’s not that important for us outdoor enthusiasts.
UTM stands for Universal Transverse Mercator.
For GPS compatible maps, like the one we have in front of us, we have a UTM system that divides the earth into a perpendicular set of grid lines.
It this image, we have a UTM northing value of 4 456 000 meters north from the equator.
The UTM grid is a kilometric grid, which means, between each corner of the square we have a 1-kilometer difference (which equals 1000 m).
That’s all to understand the different elements that come in USGS maps.
Like I’ve said before, if you master this, you can understand any other map type or even maps from other countries.
Why multiple systems to locate positions?
If you have questions like, why we have similar things like the longitude and latitude grid, the State Plane Coordinate System, and UTM system at the same time if all of them serve to locate positions.
I want you to know that these topo maps are not used by us hikers or outdoor enthusiasts only. No, it used also by local civil engineers, army, mining companies, architects, etc. So, everyone will use a different geographic grid system that suits him.
Another thing that I want you to know if you don’t see the utility of some elements, like the declination diagram, is that in this article we did just introduce you to the different elements of a map sheet, this is just an introduction.
In the next one, I will re-explain anything in detail before using it.
And if you still don’t understand something, you have the comments section below to ask your questions.