Digital Orthophoto Quadrangle (DOQ)

Digital Orthophoto Quadrangle (DOQ) data consists of a computer-generated image of an aerial photograph. The image is corrected so that camera tilt and terrain relief don’t affect the accuracy. DOQs combine the image characteristics of a photograph with the geometric qualities of a map. The USGS has DOQs available for the entire United States. Most are grayscale, infrared photos; there are higher-resolution color photos for a few large U.S. metropolitan areas.

A booming business provides high-resolution, color aerial photographs to individuals, government agencies, corporations, and educational and nonprofit organizations. Companies like AirphotoUSA (www.airphotousa.com), Keyhole (www.keyhole.com) and DeLorme’s TopoBird subsidiary (www.topobird.com) provide imagery with quality and resolution that’s close to what was only available to intelligence agencies. If you want aerial photographs for business or government purposes, check these commercial sources.

Who is Mr. Sid?

This question is more accurately stated as what is MrSID? MrSID is the Multi-Resolution Seamless Image Database. It’s a file format used for distributing large images over networks, originally developed by a company called LizardTech. Graphics in MrSID format are compressed with a lossless compression algorithm (a method of compressing data that guarantees the original data can be restored exactly) designed to produce relatively small, high-resolution images. The file format is perfect for aerial and satellite images that have large file sizes, and the government is increasingly using it for distributing data. (The Library of Congress is even using it for electronic versions of paper documents.) A number of free viewers support MrSID; use Google to find download sites. (One of my favorites is IrfanView, which is available at www.irfanview.com.)

Digital Raster Graphics (DRG)

Digital Raster Graphics (DRG) data is a scanned image of a USGS topographic map. These digital maps are available for free on the Internet or are sold commercially in collections on CDs or DVDs.
These digital maps are scanned at 250 dpi (dots per inch) and stored in a TIFF file format, using embedded GeoTIFF (geographic information) tags for location data.
You can view the map by itself or both the map and its location data. Use one of the following methods:

• View the map by opening the DRG file with any current graphics program that supports large TIFF files.
• Use the DRG file with a mapping program that supports GeoTIFF to view the map and access its location data.

For more technical details about USGS digital map data, check out the agency’s product Web site:
mapping.usgs.gov/products.html

Elevation data

You’re likeliest to run into one of these two main data formats used to represent elevation:

• Digital Elevation Model (DEM): The USGS uses this raster data format to record elevation information (based on topographic maps) and create three-dimensional representations of the terrain.
• National Elevation Dataset (NED): This format shows digital elevation data in shaded relief. It’s designed for seamless coverage of the United States in large raster files.

What is TIGER?

The U.S. Census Bureau produces Topologically Integrated Geographic Encoding and Referencing (TIGER) data for compiling maps with demographic information. This vector data is a primary source for creating digital road maps of the United States.
TIGER data is free (check out the Census Bureau Web site at www.census.gov/ geo/www/tiger/index.html) but in some areas isn’t very accurate; roads don’t appear on the map and addresses aren’t in the right locations. The government is improving the accuracy of the dataset, and plans to provide better data in the future. You’re better off using some of the free and commercial street map programs and Web sites.

Looking at Map Symbols

Symbols — icons, lines, and colored shading, as well as circles, squares, and other shapes — are important parts of a map’s language. They give the map a more detailed meaning without cluttering up the picture with too many words. They represent roads, rivers, railroads, buildings, cities, and just about any natural or man-made feature you can think of.

Symbols are either shown on the map or are compiled in a separate map symbol guide. Whether you’re using a paper or a digital map, always familiarize yourself with its symbols. The more symbols you know, the better decisions you make when you’re relying on a map for navigation.

Map symbols aren’t universal. A symbol can have different meanings on different maps. For example, the symbol for a secondary highway on a USGS topographic map is a railroad on a Swiss map.
These Web sites show what symbols for different types of maps mean:

• Topographic maps http://mac.usgs.gov/mac/isb/pubs/booklets/symbols
• Aeronautical charts www.naco.faa.gov/index.asp?xml=naco/online/aero_guide
• Marine charts http://chartmaker.ncd.noaa.gov/mcd/chart1/chart1hr.htm

Measuring Map Scales

Most maps have a scale — the ratio of the horizontal distance on the map to the corresponding horizontal distance on the ground. For example, one inch on a map can represent one mile on the ground. The map scale is usually shown at the bottom of the map in the legend.

Often, rulers with the scale mark specific distances for you. Many maps use a representative fraction to describe scale. This is the ratio of the map distance to the ground distance in the same units of measure. For example, a map that’s 1:24,000 scale means that one inch measured on the map is equivalent to 24,000 inches on the ground. The number can be inches, feet, millimeters, centimeters, or some other unit of measure. The units on the top and bottom of the representative fraction must be the same. You can’t mix measurement units.

When you’re dealing with scale, keep these guidelines in mind:

• The smaller the number to the right of the 1, the more detail the map has. A 1:24,000 map has much more detail than a 1:100,000 scale map. A 1:24,000 map is a large-scale map, showing a small area.
• The smaller the number to the right of the 1, the smaller the area the map displays.

All sorts of rulers and measurement tools are calibrated to scale for measuring distance on paper maps. Mapping software makes distances easier and quicker to determine by offering tools that draw a line between two points and show an exact distance.

Township and Range Mapping

The Township and Range coordinate system has been used since the 1790s to survey public lands in the United States. Technically, the official name of this system is the Public Land Rectangular Survey (PLS), but in practical use, most people call it Township and Range.

This coordinate system was developed after the American Revolution as a way to survey and grant title to land that was newly acquired following the country’s independence. Thomas Jefferson helped develop the system, which was enacted under the Northwest Land Ordinance of 1785. Township and Range isn’t used in the eastern United States (or in a few other states) because land surveys in those states had been completed. The system is based on the following components:

• Meridians and baselines. These lines are the foundation of the Township and Range system:
• Meridians are imaginary lines that run north to south.
• Baselines are lines that run east to west.
• An initial point is where a meridian and a baseline meet. The California Bureau of Land Management has a nice online map of all the meridians and baselines at www.ca.blm.gov/pa/cadastral/ meridian.html.
• Townships: Townships are the horizontal part of the coordinate system.
• Each township is six square miles in size.
• Townships are identified by whole numbers starting with 1.
• The first township at the intersection of a meridian and baseline is 1, the next township is 2, and so on.
• If a township is north of the baseline, it’s identified with an N; if it’s south of the baseline, it’s designated with an S. For example, the fifth township north of a meridian and baseline is T. 5 N.
  
• Ranges: Ranges are the vertical part of the grid scale.
• Ranges are six miles wide.
• Ranges are numbered starting at the intersection of the meridian and the baseline.
• In addition to a number, a range is identified as being east or west of a meridian. For example, the third range west of the meridian and baseline is R. 3 W.

The intersection of a township and range (a 36-square mile parcel of land) also is also called a township. This bit of semantics shouldn’t have an effect on you using the coordinate system, but watch out for someone else doing this.
Like other coordinate systems, Township and Range uses smaller measurement
units to identify a precise location. These units include

• Sections: A 36-square-mile township is further divided into 36 one-mile squares called sections. Sections are numbered 1–36. Number 1 starts in the top, right of the township, and the numbers sequentially snake back and forth across the section, ending at number 36 in the bottom-right corner.
  
• Quarters: Sections are divided even further by slicing them into quarters.
• Quarters are identified by the part of the section they occupy, such as northwest, northeast, southwest, or southeast.
• You can further narrow the location with quarter quarters or quarter quarter-quarters.

Township and Range coordinates are a hodgepodge of abbreviations and numbers that lack the mathematical precision of latitude and longitude or UTM. For example, the Township and Range coordinates of Dillon Falls are

SE ¼ of SW ¼ of NE ¼, Sect. 4, T. 19 S, R. 11 E, Willamette Meridian

To describe a location with this coordinate system, you start from the smallest chunk of land and then work your way up to larger chunks. Some people ignore this convention and reverse the order, skip the meridian, or use both halves and quarters. (Hey, it keeps life interesting. . . .) Although scanned paper maps (such as USGS topographic maps) often show township and range information, most digital mapping software and GPS receivers don’t support township and range. This is good news because latitude and longitude and UTM are much easier to use. Township and range information usually is omitted from digital maps because

• The coordinate system is difficult to mathematically model.
• Townships and sections may be oddly shaped because of previously granted lands, surveying errors, and adjustments for the curvature of the earth.

Peter Dana’s comprehensive Geographer’s Craft Web site has lots of good technical information on coordinate systems:
www.colorado.edu/geography/gcraft/notes/coordsys/coordsys.html

Specialized coordinate systems

Here are a few other coordinate systems so you know what they are:

• MGRS (Military Grid Reference System): A coordinate system used by the U.S. and NATO military forces. It’s an extension of the UTM system. It further divides the UTM zones into 100-kilometer squares labeled with the letters A–Z.
• State Plane Coordinate System: A coordinate system used in the United States. Each state is divided into at least one State Plane zone. Similar to the UTM system, it uses feet instead of meters.
• Proprietary grids: Anyone can invent a coordinate system for finding locations on a map. Examples of proprietary systems are ZIP code, the Maidenhead Locator System (a grid system for amateur radio operators) and Thomas Brothers street guides (that match a location with a page number and grid).

Most coordinate systems try to make navigation and surveying more accurate and simpler. GPS is sending less-used coordinate systems the way of the dinosaur because you can quickly and easily get precise location positions in either UTM coordinates or latitude and longitude with an inexpensive GPS receiver.

Universal Transverse Mercator (UTM)

Universal Transverse Mercator is a modern coordinate system developed in the 1940s. It’s similar to latitude and longitude, but it uses meters instead of degrees, minutes, and seconds. UTM coordinates are very accurate, and the system is pretty easy to use and understand.

Although the United States hasn’t moved to the metric system, the system is widely used by GPS receivers. UTM coordinates are much easier than latitude and longitude to plot on maps. The two key values to convert metric measurements are

• 1 meter = 3.28 feet = 1.09 yards. For ballpark measurements, a meter is a bit over a yard.
• 1 kilometer = 1,000 meters = 3,280 feet = 1,094 yards = 0.62 miles.

For ballpark measurements, a kilometer is a bit more than half a mile. The UTM system is based on the simple A, B, C/1, 2, 3 coordinate system. The world is divided into zones:

• Sixty primary zones run north and south. Numbers identify the zones that run north and south.
• Twenty optional zones run east to west.

These zones indicate whether a coordinate is in the Northern or Southern Hemisphere.
Letters designate the east/west zones.
Often the letter is dropped from a UTM coordinate, and only the zone is used to make things simpler. For example, because most of Florida is in Zone 17 R, if you were plotting locations in that state, you could just use Zone 17 in your UTM coordinates. To provide a precise location, UTM uses two units:

• Easting: The distance in meters to the east from the start of a UTM zone line The letter E follows Easting values.
• Northing: The distance in meters from the equator The letter N follows Northing values.

There’s no such thing as a Southing. Northing is used in the Southern Hemisphere to describe the distance away from the equator, even though a location is south of the Equator. (Is that weird, or what?) Continuing with my example of Dillon Falls, if you use UTM to locate the falls, the coordinates look like this:

10T 0627598E 4868251N

That means that the falls are in Zone 10T, which is 4,868,251 meters north of the equator and 627,598 meters east of where the zone line starts. (For those of you without a calculator in front of you, that’s about 3,025 miles north of the equator, and about 390 miles east of where the number 10 Zone line starts out in the Pacific Ocean.)

What is Longitude?

Longitude works the same way as latitude, but the angular distances are measured east and west of the prime meridian (which marks the 0-degrees longitude line that passes through Greenwich, England, without even disturbing traffic).

• When you travel east from the prime meridian, the longitude increases to 180 degrees.
• As you go west from the prime meridian, longitude also increases to 180 degrees. (The place where the two 180-degree longitudes meet is known as the International Date Line.)
• In the Eastern Hemisphere (which is east of the prime meridian to 180 degrees east), the longitude is given in degrees east.
• In the Western Hemisphere (which is west of the prime meridian to 180 degrees west), longitude is expressed in degrees west. One degree is actually a pretty big unit of measure. One degree of latitude or longitude is roughly equal to 70 miles.

Degrees are composed of smaller, fractional amounts that sound like you’re telling time.

• Degree: A degree comprises 60 minutes. One minute is about 1.2 miles.
• Minute: A minute is composed of 60 seconds.
  
• One second is around .02 miles.

These measurement units are abbreviated with the following symbols:

• Degree: °
• Minute: ‘
• Second: “

If you use minutes and seconds in conjunction with degrees, you can describe a very accurate location.
These distances are measured at the equator. At higher latitudes, the distance between longitude units decreases. The distance between latitude degrees is the same everywhere.
If you are using latitude and longitude to locate Dillon Falls on a map of the Deschutes River in Oregon, its coordinates are
43° 57’ 29.79” N 121° 24’ 34.73” W

That means that Dillon Falls is

• 41 degrees, 57 minutes, and 29.76 seconds north of the equator
• 121 degrees, 24 minutes, 34.73 seconds west of the prime meridian