Topographic map scales and the General Land Office Grid
By Rebecca L. Hylland
Maps. We all have used them at one time or another. They are sometimes stuffed into glove compartments, backpacks, and pockets. Maps get folded, crumpled, torn, and written on. I have even heard of maps going through the laundry. These used and abused sheets of paper are fairly important. Sometimes the only way we know how to “get there” is when we have a map as our guide. Maps, however, are only useful to us when we understand their language.
Since the Utah Geological Survey (UGS) began selling topographic maps in January 1994, map-related questions are a daily occurrence. Some people are curious about various map symbols and others need help with map orientation. Most of the questions we receive regarding topographic maps are about the difference in map scales, and what are “township, range, and section”, technically referred to as the Government Land Office Grid System.
Map scale is the relationship between distance on a map and the corresponding distance on the ground. Scale is expressed as a ratio, such as 1:24,000, and shown graphically by bar scales marked in feet and miles, or in meters and kilometers.
Maps with a small bar scale for example, 7.5-minute maps, are often called large-scale maps because they show more detail (by covering less area) than a large bar-scale (30- x 60-minute) map. To explain it another way, the smaller the ratio (1:24,000) the larger the scale, and vice versa. Minute refers to one minute of latitude and longitude. A 7.5-minute map covers 7.5 minutes of latitude and 7.5 minutes of longitude.
7.5-minute Map Series
The 7.5-minute map series is the most detailed available at the UGS. This series is useful because topographic features are easy to identify, which allows for accuraltely determing your location. Also, the clarity of topographic features makes route-finding easier. For these reasons, the 7.5-minute series is very popular and is typically used for hiking, hunting, geologic prospecting, locating real estate – the number of uses is endless. The main drawback of this map series is that several maps may be needed to provide coverage for a large area. One map covers approximately nine miles in a north-south direction and approximately 6.5 miles east-west.
Scale: 1:24,000; 1 inch represents 2,000 feet; 1 centimeter is approximately 240 meters; Size (latitude x longitude): 7.5 x 7.5 minutes; Minimum area covered (square miles): 49
15-minute Map Series
Also suitable for recreational and other activities is the 15-minute map series, which provides enough detail to determine location with good accuracy. These maps cover about four times the area of one 7.5-minute map. The U.S. Geological Survey has discontinued publishing and distributing this map series, but the Natural Resouces Map & Bookstore stocks over 100 different 15-minute map titles.
Scale: 1:62,500; 1 inch represents approximately 1 mile; 1 centimeter is approximately 625 meters; Size (latitude x longitude): 15 x 15 minutes; Minimum area covered (square miles): 197
30- x 60-minute Map Series
The 30- x 60-minute map series is useful for four wheeling, hunting, and other activities for which coverage of a large area, rather than detail, is required. Approximately 42 7.5-minute maps cover the area of one 30- x 60-minute map. This is a multi-purpose map series which shows most of the same features (for example, railroads, powerlines, unimproved roads, and highways) on larger scale maps, but with much less detail.
Scale: 1:100,000; 1 inch represents approximately 1.6 miles; 1 centimeter is approximately 1 kilometer; Size (latitude x longitude): 30 x 60 minutes; Minimum area covered (square miles): 1,568
U.S. 1:250,000 Map Series
This map series is the smallest scale available at the UGS. These maps show a large area; only 13 1:250,000 maps are needed to cover the state of Utah (full state coverage using 7.5-minute map requires 1,512 maps). The 1:250,000 maps series is useful for an overall view of an area and is not recommended for back-country use because of the lack of topographic detail.
Scale: 1:250,000; 1 inch represents approximately 4 miles; 1 centimeter is approximately 2.5 kilometers; Size (latitude x longitude): 1 x 2 degrees; Minimum area covered (square miles): 4,580
Township, Range, and Section
In 1812, the U.S. Government formed the General Land Office (renamed the Bureau of Land Management in 1946) to create a standardized system to more accurately define a given U.S. location. This system was initiated in response to the aquisition of large tracts of land, like the Lousiana Purchase, during the early 1800s.
The system developed by the General Land Office is called the Government Land Office Grid System and is used in states west of Ohio. This system divides land into 36 square mile units called townships. Each township has a township and range designation to define its 36 square mile area. Township is numbered north or south from a selected parallel of latitude called a base line and range, is numbered west or east of a selected meridian of longitude called a principle meridian.
Townships are sub-divided into 36 1 by 1 mile parcels called sections. Sections are numbered from 1 to 36 for identification. Sections are broken into quarters, which are further quartered to describe a property location.
The office grid system is used for legal land descriptions. As an example, the legal description for the Utah Geological Survey’s former office on Foothill Drive is written: SE1/4NW1/4, section 23, T.1 S., R.1 E., of the Salt Lake Base Line.
Glad You Asked article, Survey Notes, v. 27 no. 3, August 1995