Geology Reference
In-Depth Information
Fig. 2.1.
UTM grid (NAD-27) in north-
central Alabama as given on a
USGS topographic map. Each
square block is 1 000 m on a
side. Block 1 954 is
shaded
and Sale 1969; Snyder 1987). The UTM grid is used at lower latitudes and the USP grid
is used in the polar regions. The UTM grid divides the earth into 6
8° quadrilaterals
that are identified by reference numbers and letters. All UTM coordinates are with
respect to a survey datum that should be specified. In North America, for example,
UTMs are referenced to either the North American Datum of 1927 (NAD-27) or the
datum of 1983 (NAD-83) (Bolstad 2002).
UTM coordinates along the margin of a United States Geological Survey (USGS)
map are given at intervals of 1 000 m (Fig. 2.1). On a topographic quadrangle map, the
first digit or digits of the UTM coordinates are shown as a superscript and the last
three zeros are usually omitted. Locations within the grid are given as the coordinates
of the southwest corner of a block within the grid system. The
x
value is called the
easting, and the
y
value is called the northing. For example, the lower left coordinate
in Fig. 2.1,
5
18, is 518 000 m east of the origin. Any block can be subdivided into tenths
in both the
x
and
y
directions, adding one significant digit to the coordinates of the
sub-block. Sub-blocks may be similarly subdivided. UTM coordinates are commonly
written as a single number, easting first, then the northing, with the superscript and
the trailing zeros omitted. For example, a grid reference of 196 542 in the map of Fig. 2.1
represents a block 100 m on a side with its southwest corner at
x
= 196,
y
= 542. A grid
reference of 19 605 420 represents a block 10 m on a side with its southwest corner at
x
= 1 960,
y
= 5 420. This coordinate system is internally consistent over large areas and
is convenient for maintaining large databases. The United Kingdom uses a similar metric
system called the National Grid (Maltman 1990).
The surface locations of wells are commonly recorded according to the coordinates
given on a cadastral map, which is a map for officially recording property boundaries,
land ownership, political subdivisions, etc. In much of the United States the cadastral system
is based on the Land Office grid of Townships and Ranges (Fig. 2.2). The grid is aligned
with latitude at base lines and with longitude at guide meridians. Every 24 miles the grid
is readjusted to maintain the 6 mile dimensions of the blocks. An individual township is
located according to its east-west coordinate (Township) and its north-south coordinate
(Range). The township that has been subdivided (Fig. 2.2) is T.2N., R.3E. A township is
subdivided into 36 sections, each 1 mile on a side and numbered from 1 to 36 as shown in
Fig. 2.2. For a more precise location, each section is subdivided into quarter sections (may
be called corners) as in the northeast quarter of sect. 7, abbreviated NE¼ sect. 7. Quarter
sections may themselves be divided into quarters, as in the northwest quarter of the
northeast quarter of sect. 7, abbreviated NW¼NE¼ sect. 7, T.2N., R.3E. Locations within
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