Description
Description
Details
Direction
Distance
Designation
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The blank area around the edge of the map is called the “Margin” or “Collar.” The information in the margin will give you the description of the map. The margin contains much of the information you need to interpret the map.
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In the margin of topo maps there is a Title Bar on the top right or top middle and shows the Quadrangle the map represents. Below is the “Ferndale” quadrangle map title bar. Sometimes the title bar or parts of the margin shows the name of the quadrangle next to the map you're looking at.
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In the image below, the picture of Arkansas shows where this map is in the state. Below that is a 3x3 grid with the current map in the center (in red). The surrounding squares list the names of the quadrangle maps that surround this current map. For example, to the south is the "Congo" quadrangle (box 7).
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Maps are made at different times in history and the items on the map that were there years ago, may be gone now. So a 1945 map could be very different than a 2022 map. You need to watch the date of a map for this reason. If the map is old, some of the land features may have changed. The date is on the bottom right of map or in the title. In this map, the date is 2020 (see image above).
The map Datum (not "date") describes the rules used to the create map and is displayed in the lower left. The datum will also show the UTM Zone the map represents (more later).
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What is meant by "rules"? The rules describe what system or controls the surveyors and cartographers used to determine distances and directions. In other words, where is the baseline, reference, or "zero" from which everything was measured. When you use a ruler or tape measure, don't you need to align the 0 mark with your starting point? So do surveyors and cartographers. How they find that "0" are the rules this refers to?
Two of those "rules" are the NAD which stands for North American Datum and the WGS which stands for World Geodetic System. The two numbers that follow these abbreviations refer to the year it was created. NAD and WGS were the rules used to determine the position, i.e. latitude/longitude, of land masses and objects on maps.
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NAD 27- North American Datum of 1927. This is an older rule system that used a ranch in Kansas (Meade Ranch) as a "zero" point and about 2,600 other survey markers around the country. From those points, by surveying and triangulation, the surveyors who made the map determined the position information for land and water masses, and various landmarks. Since the distances between points lie along the surface of the Earth, these are referred to as horizontal measures. So Latitude and Longitude coordinates are "horizontal measures" and are found with NAD.
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NAD83- North American Datum of 1983. This is a newer system that used the center of a mathematical model of the Earth for its "zero" point and about 250,000 survey markers around the country. NAD83 only provided horizontal position information like NAD27, but more accurately.
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You'll notice that USGS topographic maps also provide elevation information. The "zero" or vertical reference point for elevation measurements is taken as Mean Sea Level (MSL). The NAD systems do not determine elevation information. But NAVD88 (and WGS) do.
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NAVD88- North American Vertical Datum of 1988. This rule defines the reference Mean Sea Level (MSL) at a single station in Quebec Canada. This location defines a "zero" point for elevation. This rule only provides elevation information. The method a topographic map displays this elevation is either with lines showing points of equal elevation or with shading. More on this later.
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WGS84- World Geodetic System of 1984. Similar to NAD83 in how it uses a mathematical model of the Earth and defines its reference at the center of the Earth, WGS84 provides both positional information and elevation information. This is the standard datum used in GPS. It's accurate to better than 2 cm.
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A new datum is being developed that is modernized with the use of GPS, but hasn't been released yet.
If you use multiple maps (including maps in GPS), make sure the datums of the maps are the same. Otherwise, coordinates may not transfer properly.
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Included in the margin is:
1) The date it was created. Usually on the lower right.
2) Names of the maps that are immediately adjacent to the map you're looking at (shown in the center). A 9 panel grid to the left of the date.
3) Contour interval (the height difference between two adjacent contour lines). Above the bar scales.
4) Some map symbols. Normally on the lower right.
5) The directions of True North (★ or TN), Grid North (GN), and Magnetic North (MN). To the right of the datum.
6) The map's scale (numeric ratio and bar scales). In the bottom center.
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These will be discussed in further detail in the remaining sections.