Wilderness Land Navigation - Topographic Maps 101
In today's post we will cover the basics of Topographic Maps. We will cover what is a Topographic Map as well as what information can be found on a topographic map and how that information is displayed on that map. This will be a fairly detailed explanation but I will attempt to keep the information easy to understand.What is a Topographic Map?
Topographic maps show terrain, or the "lay" of the land, using contour lines. Topographic maps also show bodies of water and various man made structure such as buildings, forest cover, and roadways. Contour lines are curves printed on the map that connect points of the same altitude on a given terrain feature such as a mountain. Topographic maps are also sometime referred to as "topo" maps. In the U.S. topographical maps are typically organized in a series of 7.5 minute grids and they are sometimes referred to as "quadrangles" or more simply as "quads". These maps are produces to cover an area of 7.5 minutes latitude by 7.5 minutes longitude (typically 49 to 71 square miles) and are printed to a scale of 1:24,000 where 1inch(map) represents 2000 feet(land) or 1 centimeter(map) represents 240 meters(land). Interestingly, the north and south sides of these maps are not straight lines, but are actually slightly curved to match the lines of latitude of the Earth. Also, the east and west sides of the maps typically not parallel as they match the lines of latitude of the Earth.What Information can be Found on a Topographic Map?
We will now cover what information is contained on the map sheet. This portion will be covered into three parts, information on the map margin, terrain features, and colors used on the map.Information on the Map Margin.
Most topographic maps will having information printed around the margins of the map sheet. This information can be thought of as the "User Manual" for the map. As with any other piece of your kit it is important to read and understand the information contained in the manual to get the most efficient use of the tool and to use it safely. Topographic maps should be considered a tool and treated and cared for as such. Many people have difficulty using a map simply because they do not understand the information shown on the map, and this can be solved by reading the instructions provided.
On the top left of all 7.5 Minute maps produced by the USGS is the name of the organization responsible for creating the map. In this case it is the USGS.
On the center portion of the top of the map will be listed either the map series as with the newer US Topo maps, or the secondary Agency responsible for information on the map.
The top right corner will show the complete quadrangle name along with the State and often the County in which the area represented on the map is located. Also shown will be the are covered by the map, here listed as 7.5-Minute Series.
At the bottom right will be found the map key, showing how roads and other features are indicated on the map. The map name and State are also shown along with the date the map was produced. This date is one of the major key features to look for on all maps. Also shown is a representation of the maps location within an outline graphic of the State. Most newer maps will have a nine square grid in this are also indicating the adjoining map sheets. This information is helpful when navigating near the edges of a map sheet.
At the bottom center the Map Scale will be found. Included with the scale are distance bars to aid in measuring distances on the map, and will be shown in kilometers, meters, miles, and feet. Additionally shown are two vital pieces of information, contour interval and map datum. The contour interval is the vertical distance between the brown contour lines on the map. Many intervals are used by the USGS from 10 feet in relatively flat areas to 100 feet in very mountainous areas, but typically 20 to 40 feet intervals are most common. The Datum that was used to produce the map is simply an agreed upon starting point for measuring locations on the Earth. Usually a map will be produced with North American Datum of 1983 (NAD83) or, on older maps North American Datum of 1927 (NAD27). These are important when communicating coordinates to persons using other maps or when using a GPS in conjunction with a map. Utilizing different datum sets can cause significant differences in the actual locations of a given coordinate.
The bottom left corner of the sheet contains a lot of information and is easily confusing. Here we will find the "Declination Diagram", this diagram indicates the difference between Magnetic North (Compass North), True North (North Pole), and Grid North (Top of Map Sheet). It is easy to get hung up on these but we will discuss this later. Typically the difference between True North (TN) and Grid North (GN) is negligible and True North is not typically need for wilderness land navigation. Newer maps will show the UTM Grid Zone that the map is located in. In wilderness land navigation UTM (Universal Transverse Mercator), MGRS (Military Grid reference System), and the USNG (United States National Grid) are functionally the same when used in any of the States. Again, this will be covered in a later post in detail. The other information is credit information, think of it as the bibliography of the map. This shows what sources of information were used in creating the map.
As you can see, there is a wealth of useful and important information located in the margin of the map sheet. This information will make it much easier to understand what is being represented on the field of the map sheet.
How is Information Displayed on a Topographic Map?
Information is displayed on a topographic map by the map makers utilizing various lines and designs along with several colors in order to represent the elevation change of an area as well as the features contained in that area. The most prominent feature on a topographic map is the contour lines. Contour lines do not exist on the land in reality, but are used to indicate a set of contiguous points in an area that are the same elevation. Learning to recognize and visualize what patterns the contour lines form will make a better a safer wilderness land navigator. Many different symbols are also used to indicate features and locations on the map. A complete guide to colors and symbols contained on USGS topographic maps can be found here: http://pubs.usgs.gov/gip/TopographicMapSymbols/topomapsymbols.pdf and here: http://nationalmap.gov/ustopo/images/US_Topo_Map_Symbols.pdf . These two document provide a very valuable resource when studying topographic maps for wilderness land navigation and I strongly recommend becoming familiar with them.Terrain Features.
Terrain features on the ground will be represented by the pattern that is formed by the contour lines shown on the map. Understanding how these features are represented on the map can make it easier to plan a comfortable efficient route while navigating in the wilderness. It is important to remember that all land features may not be represented on the map. For example a deep gouge or ditch may not be shown because it may lie between two contour lines and not cross them. Care should be taken while navigating as to not stumble into an area not indicated on the map. Terrain features indicated on the map can be divided into five major and five minor features. Many features can be better understood by utilizing your hand to indicate the features.The Five Major Terrain Features:
Hill - a single point rising in elevation above the surrounding land. A hill can be visualized as a single knuckle on a clinched fist.
Ridge - A ridge is simply two or more hills connected in a line. A ridge can be visualized by the four knuckle of a clinched fist.
Saddle - A saddle is the low area connecting to hills. A saddle can be visualized as the are between two knuckle on a clinched fist.
Valley - A Valley is the lowest area between two ridges. Rivers and large creeks are often found in a valley. The valley can be visualized as the area between two extended fingers on you hand, the fingers represent the ridges and the are between is the valley.
Depression - A depression is an area near gently sloping to flat land that is slightly lower than the surrounding land. A depression will be indicated as a contour line with small tick marks pointing toward the lower elevation. Think of a depression as a pond or lake that does not contain water. A depression can be visualized by the cupped palm of the hand.
The Five Minor Terrain Features:
Spur/Point - A spur or point is the area of land extending down grade from a hill top. A spur or point will often form a "U" shape in the contour lines and the bottom of the "U" will point to the lower elevation. A spur or point can be visualized by the finger in a clinched fist where the finger indicates lowering elevation from the knuckle or hill.
Draw/Hollow - A draw or hollow is the area of lower elevation between two spurs or points. Water can often be found in a draw or hollow in the eastern woodlands. A draw or hollow will form a "V" in the contour lines where the bottom of the "V" points to higher elevation. A draw or hollow can be visualized by the area between two fingers in a clinched fist. Where the fingers represent spurs extending down from a ridge.
Cliff - A cliff is an area of drastic elevation change in a short linear distance. A cliff can be formed naturally by erosion or other process, or can be formed by excavation of an area for road building or mining. A cliff is indicated by converging contour lines on the map and may appear as a solid brown mass due to the contour lines being very close in the area. A cliff may be visualized by the sides of the clinched fist in that the side are near vertical in nature.
Cut - Rarely indicated on a map except possibly as updated information on a map. A cut is an area that has been excavated in order to build a roadway, rail line, or possibly a large building. A cut will be indicated as missing contour lines with straight lines containing tick marks pointing toward the lower elevation.
Fill - Again a fill is rarely indicated on a map except usually in an updated map sheet. A fill is similar to a cut except a fill is an area in which land has been added in order to fill a low lying area to be level with another area. A fill will often be used in conjunction with cuts in order to build roadways or rail lines. A fill will be indicated by straight lines with small tick marks pointing to the lower elevation. Often fills and cuts will be found in close proximity to one another.
Colors Used on the Map.
Typically used on all USGS are five major colors as well as a few supplementary colors may be found on some maps.The Five Major Colors:
Brown - Contour Lines - Line close together indicate steep terrain and lines further apart indicate gentle sloping or flat terrain.
Index Contour lines will be bold and have the elevation level, usually 100 feet) indicated on the line in several locations.
Intermediate Contour lines will be narrow and will represent the contour interval as indicated at the bottom center of the map sheet.
Blue - Water Features
Green - Forest/Vegetation Cover
Red - Roadways
Black - Man Made Features, smaller roadways, and trails
Supplementary Colors:
Pink - Newly built up areas
Purple - Updated map information (when not enough to produce an entirely new map)
Yellow - The new US Topo maps will have the USNG drawn onto the map field in Yellow.
Understanding these colors will help make sense of what is being displayed on the map.
nice explanation!
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