Lecture 37 Reading a Topographic Map Today we’ll practice reading a standard USGS Topographic Quadrangle map of South Buffalo Before starting, let’s have a look at some essential map elements

USGS Topographic Quadrangle Map Legend (Notice your map is not square….why?)

Map Scale For example: On a 1:24000 quad map, scale factor = 24000, so a distance of 1cm on the map represents a true distance of 24000cm or 240m. Note: Your map has been rescaled to fit on an 11’’ x 17’’ sheet of paper, so it’s no longer 1:24000! The scale bar, however, is still accurate.

Map Projection

Map Projection The Datum: A mathematical model of the Earth (Basis for NAD83 and WGS84)

The Transverse Mercator Projection Map Projection The Transverse Mercator Projection Tangent Case Secant Case Yellow zone bounded by two standard meridians and one central meridian Secant Case A conformal projection, meaning local angles and shapes are preserved.

The Transverse Mercator Projection Each UTM zone = 6 degrees of longitude

Universal Transverse Mercator (UTM) Zone Coordinate System (-,+) (+,+) Central Parallel (-,-) (+,-) Northings Eastings “False Origin” Central Meridian Northings/Eastings are reported in meters

Universal Transverse Mercator (UTM) Zone Coordinate System Example: Easting 675000m, Northing 4750000m (Note orange UTM grid: 1000mx1000m)

True North, Magnetic North, and Grid North

True North vs. Magnetic North Earth exhibits a magentic field roughly analogous to a magnetic dipole However, the poles of the magnet do not coincide with the geographic poles (i.e., the spin axis) At any point on the planet, a compass reading (which measures the magnetic field) will show magnetic north, which in most places is offset from true (geographic) north

True North vs. (UTM) Grid North GN

Topographic Contours Contours join lines of equal elevation above a datum ( ≈ sea level) Provide a 2D representation of a 3D surface

Topographic Contours Landscape Perspective View (3D) Topographic Cross-Sections Topographic Contour Map (2D)

Topographic Contours Contours join lines of equal elevation above a datum (≈ sea level) Provide a 2D representation of a 3D surface

Making a topographic profile - Any arbitrary traverse across a topographic map can be converted to a topographic profile (i.e., a plot of elevation)

Topographic Contours Resolution (level of detail) depends on contour interval; scale can be adjusted to give the desired vertical exaggeration Elevation Distance

Traverse From Map Elevation Profile Elevation (ft) Distance 860 840 820 800 780 Distance (Scale = Map Scale)

Topographic Contours Note: Both plots convey the same information, but the vertical exaggeration affects the appearance of the plot VE=Vertical Scale/Horizontal Scale H2 Elevation High Vertical Exaggeration H1 Distance H2 Low Vertical Exaggeration Elevation H1 Distance

Surface Hydrology and Topography

Have you noticed a relationship between topography and watercourses (i Have you noticed a relationship between topography and watercourses (i.e., creeks and rivers)?

Drainage Basins (or “Catchment Areas”) - The “collection area” for water that falls on the Earth’s surface - Defined above any point on a stream Drainage Area Above Point P Separated by drainage divides Not fixed over geologic time “Nested” P

Drainage Basins of W NY Example of a regional drainage divide crossing the Ellicottville area

Ellicottville Quad To Cattaraugus Ck… To Allegheny River… From the pattern of the streams, we can make out the regional drainage divide

Ellicottville Quad To Cattaraugus Ck… C B To Allegheny River… A D From the pattern of the streams and topography, we can map surface drainage in great detail

A B Elevation Distance C D Elevation Stream Stream Distance

Ellicottville Quad To Cattaraugus Ck… To Allegheny River… From the pattern of the streams and topography, we can map surface drainage in great detail

Stream Networks - Drainage areas evolve stream networks -The networks are commonly characterized by their branching patterns Drainage Area Above Point P - Stream Order indices are used to describe the branching pattern in a network 1 1 1 1 1 2 2 3 1 3 6 P

Ellicottville Quad To Cattaraugus Ck… To Allegheny River… 1 1 1 2 1 1 3 3 1 ? Example of Stream Ordering

Stream Long-Profile Just a topographic profile that traces the axis of a stream or river ? Elevation Distance Downstream