Topographic and Geologic Maps

Learn How To Use These Maps

Determining Location • Latitude and Longitude

• Public Land Survey System

Determining Elevation • “See” the Shape of Earth’s Surface

• Allows Identification of Geologic Features

Determine Rocks, Contacts and

Structures

1° = 1/360 of a

circle

1° = 60’ (Minutes)

1’ = 60” (Seconds)

On some measurements we will get to the nearest 15”

You may also see “Mils”, short for “Mil Radian”, used by

militaries (there are 2 Pi Radians in a circle, divided by 1000, and

you get 6283 mils in each circle).

This Classroom is ~ 35°24’30” N Lat, 118° 58’15” W

Lon

Now We’ll Use the Maps

Superimposed Grid over the Earth

Start with an arbitrary point of origin • Mount Diablo in the Bay Area (MDBM)

• Mount San Gorgonio near San Bernadino (SBBM)

• Mount Pierce near Humboldt (HBM)

Count each row (“Township”) north or

south

Count each column (“Range”) east or

west

Scale • Relation of map to real world

• Fractional Scale (1:24,000 or 1:62,500)

• Graphic Scale

• Verbal Scale

• Lat/Lon Scale (How many Minutes?)

Naming the Maps, Publication data

Adjacent Maps (two ways)

Magnetic Declination

Elevation relative to Mean (average) Sea Level

Depicted on “Topo” Maps with brown contour lines

Each contour is a line of equal elevation

Index Contour Line = heavier lines with labels, every

fifth line

Contour Interval = elevation change with each line (40’,

80’ most common)

Closely vs. widely spaced contours

Most of our maps are in feet, few in meters

Geology Lab

River Elevation

Canyon (note “Vs”)

Relief = High – Low elevation; total

elevation change for a map or area

Gradient = Average Slope; Rise/Run • (Elevation difference/Distance)

Topographic Profile = Graphic

representation of elevation change,

essentially a cross section of the surface

shape.

“Tick” Marks indicate decreasing elevation

Depict the rock types and structures at the surface

Show the relationships of these units (contacts)

Each map has a legend with rocks listed by age (oldest on the bottom, scale on side), and may be in the description

Each rock unit is distinguished by color and symbol

The legend depicts other symbols as well, including contacts, and structures (faults, folds)