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Frequently Asked Questions:
All About Maps

compiled by Shari Kelley
What is a topographic map?
A topographic map is a scaled, two-dimensional representation of the three-dimensional surface of the earth. The U.S. Geological Survey offers a handy booklet that defines topographic maps and describes how they are made. Topographic maps are available for sale from our main office in Socorro.
What is a LANDSAT image?
Landsat images are produced by special satellites fitted with remote-sensing equipment. Remote sensing is a process of collecting data from a distance. The sensor on each satellite is able detect distinctive types of electromagnetic radiation emitted by objects on the Earth's surface. The sensor then records these data as different wavelengths to form the image. Only two Landsat satellites are in operation now. For any given spot on the earth's surface, an image is taken every few days. The sequential images are used to monitor changes in vegetation density, urbanization, sea level, glacier size, and many other physical characteristics of the earth's surface. [Note: we have a LANDSAT image of the state of New Mexico for sale as Resource Map-23.]
What is a geologic map?
For a good discussion of geologic maps, check our 'What are geologic maps and what are they used for?' page. Our geologic maps that are available for sale or download can be accessed from our geologic maps page.
How are maps scaled?
The Earth's landscape has to be scaled down proportionally so that the resulting map fits onto a piece of paper or a computer screen that is of a manageable size, and yet shows an appropriate level of detail. Different map scales suit different needs. One commonly used map scale is 1:24,000. A map scale is a ratio that indicates that one unit of measure on a map is equivalent to 24,000 units on the ground (in this example). So, 1 inch measured on a 1:24,000-scale map will equal 24,000 inches on the ground (or 2000 feet). On a 1:100,000 scale map, 1 cm represents 100,000 cm (that's 10 km) in the real world. Since the ratio 1/100,000 is a smaller number than the ratio 1/24,000, a 1:100,000 map is a smaller scale than a 1:24,000 map. The smaller the scale, the more real-world real estate a square inch (or any unit of measure) represents.
What is meant by "7.5 minute" and "15 minute" maps?

The Earth has been divided into a coordinate system that accounts for the spherical shape of our planet. The Earth is divided along north-south lines into 360 1° arcs that resemble orange slices. The Prime Meridian (0° longitude) is defined to run through Greenwich, England; the N-S lines of longitude are numbered 0 to 180°W and 0 to 180°E of the Prime Meridian. New Mexico lies between 103 and 109°W longitude. East-west lines of latitude run from 0° at the equator to 90° at the poles. New Mexico lies between about 32 and 37°N latitude.

Each degree is divided into 60 minutes and each minute is divided into 60 seconds. One minute of latitude equals 1.86 km and one second of latitude is 30 m, but the distance between for one minute and one second of longitude is variable, decreasing poleward. In New Mexico, one minute of longitude is approximately 1.58 km in the south and 1.48 km in the north.

A 7.5 minute map is bound by 7.5 minutes of latitude and 7.5 minutes of longitude. USGS 7.5-minute quadrangles are are topographic maps produced at a scale of 1:24,000. Similarly, a 30x60-minute map encompasses 30 minute of latitude and and 60 minutes of longitude (1 degree). The USGS produces 30x60-minute maps at a scale of 1:100,000 with a metric countour interval.

The U.S. Geological Survey has an excellent discussion about map projections, the process of depicting the spherical Earth on a flat piece of paper or flat computer screen.

Where can I purchase maps?

The New Mexico Bureau of Geology Publications Office has a complete collection of topographic maps for the state available for sale. Published geologic maps are also available for purchase in our Publications Offices.

Topographic maps are often available at sporting goods stores.

The U.S. Geological Survey carries a huge assortment of topographic maps, geologic maps, and digital map products.

Where can I download digital data?

A wealth of digital geographic data for New Mexico, including topographic data, can be accessed at the New Mexico Resource Geographic Information System Program site.

General maps for New Mexico can be accessed at the Geology.com site.

The U.S. Geological Survey is a great resource for digital map data.

What is meant by Township and Range?
The Public Land Survey System, often known as just "PLSS", was developed in the early 19th century in the western and midwestern United States and in western Canada to describe the location of parcels of land. A north-south line known as the prime meridian and an east-west line known as a baseline were defined and surveyed in each state. Lines that are six miles apart were surveyed on either side of the prime meridian and the baseline, forming a grid of 6 mile by 6 mile blocks across the state. Townships are rows numbered north and south of the baseline, and ranges are columns numbered east and west of the prime meridian. Each 36 square mile block is further subdivided into one square mile sections. The one square mile sections can be divided into quarters for further refinement of the location of certain features, such as oil wells.
What is UTM?
UTM is an acronym for Universal Transverse Mercator, a coordinate system that covers the Earth. The Earth is divided into 60 zones that include 6° of longitude and 8° of latitude. Each zone is divided into a 1 km by 1 km square grid. Coordinates are reported by zone, then easting and northing with respect to the reference grid. You can also use one of our free locater grids to find UTM coordinates on a map.
What is GIS?
GIS is an acronym for Geographic Information System. A GIS is a system for management, analysis, and display of geographic data. The computer programs associated with a GIS are capable of integrating a variety of georeferenced data into a single map or set of maps. GIS has applications across a range of disciplines, including geology and geography.
What is GPS?
GPS is an acronym for Global Positioning System. Handheld GPS receivers are very useful for navigation in the field. See our separate discussion page about GPS technology.