Lake Basin Characteristics and Morphometry
3
time is related to the shifting ofthe earth's magnetic poles. Important souces of error are
annual variations, irregular variations of unknown cause, and local attractions caused
by magnetic substances in the immediate vicinity of measurements.
A small-scale diagram, called an isogonic chart, is included on detailed maps to
indicate the extent of declination of magnetic north from true north. An isogonic chart
consists of a set of roughly parallel solid lines representing lines of equal magnetic
declination. Another set of lines, which are dashed and cut across the solid lines,
indicate the annual rate of change in the amount of declination. Those lines east of the
zero (agonic) line show west declination; those west of it show east declination. Isogonic
charts are dated so that declinations may be determined for dates after the chart was
prepared, with the assistance of data from governmental or other sources.
Location within Public Lands
In 1785, a law was enacted for the purpose of subdividing public lands into townships
and sections, the townships to be 6 mF and the sections 1 mi 2 • In 1796, the original
system was amended so that sections were to be numbered in east-west rows,
alternately west and east, starting with 1 in the notheast corner and ending with 36 in
the southeast. This system remains in use today for all surveys throughout a majority of
the United States (McEntyre, 1978). Exceptions include the states of New England,
New York, Pennsylvania, and the Atlantic coastal states [cf., Hunt (1974)].
Because ofthe convergence of all true north-south lines (meridians), it is not possible
to layout a truly rectangular system of coordinates over large areas and maintain true
east-west and north-south boundaries. Therefore, all townships are not 6 mi 2 , nor are
all sections 1 mi 2 , even though field measurements have been done without error. An
attempt is made, therefore, to layout the townships and sections in such a manner as to
place the necessary corrections for convergence in predetermined rows of sections
within the townships (Fig. 1.2).
The starting point for the numbering of townships (the initial point) is a selected
intersection of a geographic meridian, called a principal meridian, and a geographic
parallel, called a base line. Secondary meridians are established east and west of the
principal meridian at intervals calculated to contain a number of full-sized townships
usually four townships or 24 mi. Such secondary meridians are designated first, second,
and so on, "guide meridians east" or "guide meridians west," depending on whether they
lie east or west, respectively, of the principal meridian. At intervals of 24 mi, or other
multiples of 6 mi, secondary parallels of latitude are established north and south of the
base line. These lines are designated first, second and so on, "standard parallels north"
or "standard parallels south" of the base line.
The principal meridian and all base lines, standard parallels, and north and south
boundaries of townships are continuous true north-south and east-west lines. Guide
meridians are true north-south lines but are continuous only for the distance between
two stndard parallels. Along the base lines and standard parallels, guide meridians (and
range lines) are offset in order to adjust for convergence.
Theoretically, the east and west sides of townships are each exactly 6 mi long, any
discrepancies being due entirely to error in measurement. Along the north and south
boundaries, only the south sides of those townships bordering on, and immediately
north of, the base line and standard parallels are fully 6 mi wide; all others are 6 mi less
the amount of convergence. The east and west boundary lines of townships are called
range lines, or ranges; the north and south boundaries are tiers, or, more commonly,
township lines.
3
time is related to the shifting ofthe earth's magnetic poles. Important souces of error are
annual variations, irregular variations of unknown cause, and local attractions caused
by magnetic substances in the immediate vicinity of measurements.
A small-scale diagram, called an isogonic chart, is included on detailed maps to
indicate the extent of declination of magnetic north from true north. An isogonic chart
consists of a set of roughly parallel solid lines representing lines of equal magnetic
declination. Another set of lines, which are dashed and cut across the solid lines,
indicate the annual rate of change in the amount of declination. Those lines east of the
zero (agonic) line show west declination; those west of it show east declination. Isogonic
charts are dated so that declinations may be determined for dates after the chart was
prepared, with the assistance of data from governmental or other sources.
Location within Public Lands
In 1785, a law was enacted for the purpose of subdividing public lands into townships
and sections, the townships to be 6 mF and the sections 1 mi 2 • In 1796, the original
system was amended so that sections were to be numbered in east-west rows,
alternately west and east, starting with 1 in the notheast corner and ending with 36 in
the southeast. This system remains in use today for all surveys throughout a majority of
the United States (McEntyre, 1978). Exceptions include the states of New England,
New York, Pennsylvania, and the Atlantic coastal states [cf., Hunt (1974)].
Because ofthe convergence of all true north-south lines (meridians), it is not possible
to layout a truly rectangular system of coordinates over large areas and maintain true
east-west and north-south boundaries. Therefore, all townships are not 6 mi 2 , nor are
all sections 1 mi 2 , even though field measurements have been done without error. An
attempt is made, therefore, to layout the townships and sections in such a manner as to
place the necessary corrections for convergence in predetermined rows of sections
within the townships (Fig. 1.2).
The starting point for the numbering of townships (the initial point) is a selected
intersection of a geographic meridian, called a principal meridian, and a geographic
parallel, called a base line. Secondary meridians are established east and west of the
principal meridian at intervals calculated to contain a number of full-sized townships
usually four townships or 24 mi. Such secondary meridians are designated first, second,
and so on, "guide meridians east" or "guide meridians west," depending on whether they
lie east or west, respectively, of the principal meridian. At intervals of 24 mi, or other
multiples of 6 mi, secondary parallels of latitude are established north and south of the
base line. These lines are designated first, second and so on, "standard parallels north"
or "standard parallels south" of the base line.
The principal meridian and all base lines, standard parallels, and north and south
boundaries of townships are continuous true north-south and east-west lines. Guide
meridians are true north-south lines but are continuous only for the distance between
two stndard parallels. Along the base lines and standard parallels, guide meridians (and
range lines) are offset in order to adjust for convergence.
Theoretically, the east and west sides of townships are each exactly 6 mi long, any
discrepancies being due entirely to error in measurement. Along the north and south
boundaries, only the south sides of those townships bordering on, and immediately
north of, the base line and standard parallels are fully 6 mi wide; all others are 6 mi less
the amount of convergence. The east and west boundary lines of townships are called
range lines, or ranges; the north and south boundaries are tiers, or, more commonly,
township lines.