How to Use a Map

Page 2

 

Marginal Information

 

In our discussion of the map, we will center our discussion on a single map sheet, such as a USGS 1:24,000 topographic map. 

 

The first thing to do with a map sheet is to lay it flat on a table and take a good look at it. You will see the center map picture in the center surrounded by a white border. This white border is called the "margin" and the information contained it the margin is called "marginal information." The marginal information is a lot of information about the specific map sheet your using, some of the symbols used on the map, and information about the area the map represents. So, let's take a closer look at the marginal information.

 

Start with the upper-right corner of the map.

 

NE Corner.JPG (30969 bytes)

Click Here to Enlarge Picture

 

 Here you will find the "sheet name." Each map sheet has a name and a map number (more on the number later), and that name is of some prominent feature on that map sheet. It could be the name of a town, mountain, river, or anything else that the designer desired, but it will be located on the map (in this case, North Bend). In most cases, you will also see the state and county that this area is located in. You will also see the "series." For a 1:24,000 scale map, it will say "7.5 minute series (topographic)." Just to the right of this info, you will see a small number and name printed diagonally. This is the map number and sheet name of the map that is just to the northeast of this map sheet (in this case Mount Si). So, if you needed the map sheet to the northeast, you would ask for that name.

 

Looking exactly at the northeast (top-right) corner of the map picture, you will see two sets of numbers. These numbers represent the Latitude and Longitude for that spot on the earth. We will get into latitude and longitude later.

 

Moving down the right side of the margin, you will see a series of numbers periodically along the edge of the map picture. These maps are used by us, the military, surveyors, and others, so various types of grid systems are integrated into the maps so they can be used by these various agencies. We are only concerned with a few of these systems, and will get into them more later.

 

At the half way point on the right margin is the name and number of the map sheet to the east, and moving to the southeast (lower right) corner of the map picture, you will see the latitude and longitude again, as well as the map sheet name and number of the next map to the southeast.

 

Just under the southeast corner of the map picture, you will find a "Road Classification" legend. 

 

SE Corner.jpg (23517 bytes)

Click to Enlarge Picture

 

This legend has several examples of road graphics that are used on this map sheet and their meanings. If you look at a road on the map and want to determine if it is hard surface, unimproved or super-highway, this is where you look. Under the legend, you again find the map name, the year the map was produced, and the map sheet number. This map sheet number is assigned only to this map sheet, just as the map name is. Exactly what those numbers mean is not important. What is important is that you can order this map by the name OR the number. 

 

To the left of this info is a small drawing of the state this map is located in, and a small black square showing the approximate coverage of this map.

 

Half way between the right and left corners of the map picture is the map name and number of the next map sheet to the south. Below this is the "scale" for this map. 

 

Bar Scales.jpg (31241 bytes)

Click to Enlarge Picture

 

What is "scale?" Scale is the consistent proportion at which the map picture represents the actual ground. In our example, a scale of 1:24,000 means that 1 inch measured on the map represents a distance of 24,000 inches measured on the ground. In the same way, 1 foot measured on the map represents a distance of 24,000 feet on the ground. Using this knowledge, look at the "bar scales" located just under the scale of the map. In our example, there are three scales: Miles, Feet, and Kilometers. These bar scales can be used to measure straight line distance (as the crow flies) on the map in any of these three scales. We will get into measuring distance on a map later.

 

Located under the bar scales is the "Contour Interval." First, let me explain the term "Contour." It's pretty obvious that the earth is not flat. Mountains cover the surface, and so, the earth has "contour." Each mountain, hill, valley, etc has its own unique contour. So a system had to be devised to tell, not only the height of the mountain, but also the shape of the mountain. For this, we use "contour lines." 

 

Contour Lines.JPG (62696 bytes)

Click to Enlarge Picture

 

Contour lines are those seemingly endless squiggly brown lines on the map picture. Each of these lines is drawn on an exact elevation on that "terrain feature." The distance between the lines is a representation of the vertical distance between the lines. The vertical distance between the lines is the "contour interval." In our example, the contour interval is 40 feet, which means that between each of those brown lines on the map to the one next to it is an elevation change of 40 feet. But, is it 40 feet up or down? We will get more into this later.

 

To the left of the bar scales is the "declination diagram."

 

Declination Diagram.JPG (9806 bytes)

Click to Enlarge Picture

 

 This diagram shows the difference between true north (pointing directly at the north pole), magnetic north (pointing directly at the magnetic north pole), and grid north (which is the vertical lines drawn on the map ). We will get more into this later.

 

To the left of the declination diagram is the "datum" used to produce the map. This is the information used to produce the map and is of no concern to us, unless we need to program it into a computer, or possibly a GPS. 

 

SW Corner.jpg (38511 bytes)

Clark to Enlarge Picture

 

At the southwest corner of the map picture is the map name and number of the next map sheet to the southwest, and also the latitude and longitude of that corner. Moving up the left side of the map picture, you will see the map name and number to the west and again those various number systems.

 

At the northwest corner of the map, you see the latitude and longitude for that corner and the map name and number for the map to the northwest. You also see the the producer of the map (USGS). 

 

NW Corner.JPG (31114 bytes)

Click to Enlarge Picture

 

Moving to the right along the map picture, you will see the map name and number for the map sheet to the north, and again the various number systems., which continue to the starting point.

 

Maps and Accessories you can buy now from Amazon

 

 

Previous Page                         Next page

Go to Page 1,2,3,4,5,6,7,8,9,10,11

Look for books and equipment for Land Navigation in the Mercantile

 

Submit a tip or technique