Tuesday, August 10, 2010

GPS Maps 106: Reading A Topo Map

[This post is part six in a multi-part series about understanding the GPS system and its coordinate grid.

The last (hopefully) in the series of posts about maps: how they're made and how to use them, I wanted to do something quite difficult (on this end) and that I hadn't tried before. Well… you saw what happened on the last post.

As it turned out, the "dummy down" policy of Google Earth [which supports their "paid version"] prevented anyone who didn't have a fairly new graphics card from viewing the presentation.

Therefore, lets attempt to do in writing that which I thought would be much easier in demostration; so, break out that copy of an actual topo map that you have in the glove box and follow along.

Ok, I'm trusting that you've come back with your map. I've got mine, which is the Devil's Throat, Nevada 1:24,000 quad. I'll be explaining using my map but yours should be almost identical in layout.

You need to know that we won't be repeating any of the basic material already covered. So, unless you already know this stuff, you cannot come into this in the middle. However the posts are still where we left them and you can review all of these "backgrounders":

Background and History

Who are you and what have you done with my real map?

In 1879 a civilian mapping company known as the United States Geological Survey (USGS) took over responsibility for mapping the country and they've been doing it ever since. They finished what Lewis and Clark only dreamed about; namely, create a detailed, uniform map covering the entire United States (CONUS).

Their best known, and most ubiquitous, product is the 1:24,000-scale topographic map – also known as 7.5-minute quadrangles or "quads." For the detail they wanted they needed to create no larger than 7.5 minute maps, which meant that they created more than 55,000 of them! [But at ten bucks apiece, they've done all right for themselves.]

At 1 inch = 2,000 feet (1:24,000) they show natural and cultural features at the detail necessary for engineering, local area planning and recreational purposes.

The USGS also sells maps of less detail such as the 1:50,000 (1 centimeter = 0.5 kilometer) and 1:100,000 (1 centimeter = 1 kilometer) useful for land management and planning but not for finding your way off the desert. Maps (called sheets) at 1:250,000 (1 inch = about 4 miles), 1:500,000 (1 inch = about 8 miles), and 1:1,000,000 (1 inch = about 16 miles) can only be used in regional and statewide planning but never for navigation.

The "Quad"

Most USGS maps are bounded by two lines of latitude and two lines of longitude. What is known as a "quad" (or even as just "a topo" or a "7.5 minute") is really a map which spans 7.5 minutes of latitude and 7.5 minutes of longitude. Conveniently, it is usually named after the most prominent feature in the quadrangle.

Since the beginning of the space age and invention of the internet there has been an almost frantic need to convert this "paper" into graphics which can be displayed. Google kicked the "fever" up to a high-pitched scream by developing "Earth" which relied heavily on (and solidified standardization of) the work of NASA and others, namely: Digital Raster Graphics (DRG).

DRGs are scanned images of USGS topo's produced by private organizations in a partnership. The scanned image can include all map collar information, or may be ordered without the collar information (clipped to the neat line) – you'll see. The image inside the map neat line can then be georeferenced to the surface of the Earth and now even overlayed on Google Earth!

Collar Information


The first thing to notice on a topographical map is the title. It is found in the top right hand corner of the map. The title for this particular map is, "Devil's Throat."

In the bottom collar, you'll see a similar section which graphically shows the location of this map within the state of Nevada, and the names of all of the adjoining maps.


You should get in the habit of always looking at the collar information before you even look at the graphics in order to see what you are dealing with. And one of the most important items is the Datum, namely, what datum was used to generate the map.

In this TOPOs case, as with most others, you can see that it was built using the "vertical datum of 1928" and "North American horizontal datum of 1927." This was similar to that used by Lewis and Clark. But, we've have many datums since then and are now into several revisions of the North American Datum of 1983.

This information explains why you may have had a lot of discrepancies between your GPS unit and TOPO maps in the past. Fortunately, there are references about how to correct for those descrepancies right there in the collar information. And while you are there, make a mental note of all the other information in this section which will be explained later.

Latitude, Longitude, and UTM'S:

The next thing that you should notice on a topographical map are the numbers running all around the outside of the map. These numbers represent two grid systems which can be used to find your exact location.

The first type is called latitude and longitude. The exact latitude and longitude is given at each corner of the map and at equally spaced intervals between the corners. You can see that there will be a fair amount of interpolation with any one point on the map. And, it's only fair to warn you that lat/lon markings were pretty much an afterthought on these maps; after all, they are called "Transverse Mercator Maps" which use another coordinate system entirely.

The coordinates in the example would be read: "36 degrees, 15 minutes north latitude; 36 degrees 30 minutes west longitude."

The second system is called UTM. These are the smaller bold numbers running along the border of the map. They create 1000 meter square blocks on the map which can be helpful in estimating true distances between points.

The UTM numbers shown would be read as: "4 million, 42 thousand meters Northing," (north of the equator) and of course you would also need to know that it was the Northern hemisphere. [We've already seen in the collar information that this map is in UTM zone 11]

Similar numbers are located on all four sides of the collar and contain both latitude, longitude, Eastings and Northings. Some interpolation is necessary for exact positions on the map; and, as you will soon learn, it's a lot easier for the UTM system – especially if you speak "metric."

Map Scale:

Map scale represents the relationship between distance on the map and the corresponding distance on the ground, and is represented in two different ways: a ratio and graphically.
The ratio scale on this map is 1:24,000. That means that 1 "anything" on the map is worth 24,000 of the same "anything" on the earth. One map inch = 24,000 'real' inches (2,000 feet, 667 yards, 0.38 miles) on the ground.

Below the ratio is a graphic scale representing distances in miles, feet and meters. Visually it can be used for quick visual estimates of distances and you can use it to notch a twig or napkin to estimate more closely.

Map Information

Township & Range:

Yet a third system of "categorizing" the land is seen on TOPO maps: the Township and Range System which divides an area into 36 mile square parcels – the red lines.

The Public Lands Survey System, as is it sometimes known, was developed as the country expanded to the west in order to help parcel out lands and allows any area to establish a base line and a principal meridian.

Then as you go to the east or west of the principal meridian, the range increases in that direction. If you go north or south of the base line, the township increases.

Perhaps you can see in the graphic, the red markings. The solid red line is marked "T17S" at the top and "R69E" on its side. This would be "Township 17 South, Range 69 East in Nevada."

Townships are then subdivided further into 36 sections (the dashed red lines) – perhaps you can see the faint "1" in this graphic – each one-mile square.

So, between the dark black 1000 meter squares of the UTM system and the red 1 mile squares of the Township system, there are red and black squares all over the map and you should be able to "estimate" pretty much any distance in either miles or meters.

Contour Lines:

One of the advantages to using a topographical map is that it shows the three dimensional lay of the land using contour lines – lines which connect points of equal elevation therefore forming a serpentiginous line following the topography. On this map they are the brown lines in groups of five, with every fifth one darker in color.

Sporadically they are labeled with the actual elevation in meters at that point, and you must extrapolate all the rest.

Notice these lines are at evenly spaced elevation which is known as the contour interval; again, found in the collar, below the map scale. For this map, the contour interval is 10 meters (around 33 feet).

That means that every time you go up or down to an adjacent brown line the elevation changes by 10 meters. On this map you can easily spot a cliff or mountain which changes elevation rapidly.

Notice how the lines define its shape being closer together at the top of the mountain where it is steeper. One of the lines is labeled 850 meters and they run all the way up to 1150 meters (blue markers added for clarity).

And you may even be able to tell that it does this elevation changing in about the same lateral distance as one of those square, black 1000 meter boxes [3,280.8399 feet or 0.621371192 miles].

According to the calculator, this represents a rise over run of 3:10, a 30% grade (slope) or 17° pitch (angle) – break out the carabiners and pitons.

Magnetic Declination:

In the lower left hand corner of topographical maps there may be a symbol called the magnetic declination or it may be simply given in the Datum section.

This is very important if you want to calibrate your compass to the TOPO map. You see, just like the "Earth isn't round,""north is not always north," and there are two north poles: geographic and magnetic. Also, we now know that the exact position of the north Magnetic Pole changes from year to year.

Therefore, not only is the declination different with every map, but that declination is NOT the same from one year to the next, AND even the rate of its change is not the same from year to year!

So ask yourself, "does it make sense to use a TOPOs declination information if it's more than a couple years old?" NO, it can get you muchas, muchas lost – in either meters or miles.

I'll tell you about the maps information and then give you a link to where you can find the REAL information. The center line with the star above represents the direction of true geographic north. The line coming off to the right represents the direction of magnetic north, to which a compass needle always points.

Depending on where you are on the earth, the angle of declination will be different; but, in the case of the Devil's Throat quad, it says that in 1980 the true magnetic north was 14° 40' off – to the east. Today (Aug 2010) it is 12° 4' off to the east. [Less because it is changing by 0° 6' westward per year – sort of.]

To the left of the true north line is the grid north line. This tells you how much the UTM grid and zone lines are offset from true north.

If your USGS map is more than a couple of years old (the declination date appears in the diagram), here’s an easy-to-use website that gives you the information you need for your specific area: Magnetic Declination Calculator

Topographical Map Symbols:

There are many other symbols on USGS topographical maps. Here are some of the most common:

Useful Links

So what do you think? There's more to understanding and reading a map than your first-grade teacher made you think, huh?

Here is a link to NOAAs web page which will give you the up-to-date magnetic declination corrections for US maps: Magnetic Declination Calculator. And here is a link for just about all else: Scripts for Calculating Everything.

The advent of the computer, the internet and now ubiquitous earth mapping and geo-coding software has even dramatically changed all the work the early mappers have done. And, the USGS is attempting to "catch up" with it by undertaking the development of a new generation of maps called "the US Topo."

I am attempting to entice some "worker bee" in the USGS to give an interview about what they are doing now and what it will look like in the future – but, until that happens, this series of posts is…



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