Leoweb

"In Surveying, a reference system for computing or correlating the results of a survey. There are two principal types of datums: vertical and horizontal." (Thompson, M.M., 1979 Maps for America, USGS, United States Government Printing, page 252.)

In addition to selecting the unit of measure, the user must specify the geodetic reference datum when converting latitude and longitude (lat/lon) coordinate pairs to PLSS values. Many GPS receivers have menu options for selecting these parameters and the user should become familiar with them. USGS topographic maps use the North American Datum 1927 (NAD27) and North American Datum 1983 (NAD83). The World Geodetic System 1984 (WGS84) datum works well for navigation and is frequently the default menu option for GPS devices.

To help understand the concept of a reference datum consider the following analogy. If it takes you 45 minutes to grocery shop, the “quantity” is 45 and the “unit of measure” is minutes. The question could be asked; how was the time determined? Did you use a "Sun Dial", an "Hour Glass", a "Wrist Watch" or perhaps an "Atomic Clock?" The method used would represent the reference frame for measuring the time elapsed, as each method would produce a different degree of accuracy and precision.

The geodetic datum identifies the reference frame (mathematical algorithm) used to derive the lat/lon coordinates. It is necessary to specify the datum when navigating to a location or spotting a location on a map. For LEO_07 conversions, NAD83 and WGS84 are virtually synonymous. If a user has WGS84 coordinates, selecting the NAD83 option will work just fine. However, the LEO user must be careful to avoid selecting NAD27 when NAD83 is called for and vice versa. Using the wrong datum can result in errors upwards of 60 meters (~200 feet).

A datum is based on a mathematical approximation of the shape of the earth called an ellipsoid. Over time, the ellipsoid algorithms have been refined to produce better models of the earth allowing for better mapping capabilities and more accurate navigation information. The NAD27 datum uses an ellipsoid called "Clarke 1866" which was derived from surface location measurements. Both NAD83 and WGS84 are more sophisticated models based on the earth’s center of mass. Additionally, the NAD83 algorithm is based on a network of control monuments located on the North American tectonic plate whereas the WGS84 model was developed with control monuments throughout the world. Both (NAD83 and WGS84) initially used the same ellipsoid and produced the same results. Over time, the models have been refined and now differ slightly. The initial WGS84 and NAD83 models used a semi-polar axis calculation of 6365752.314140 meters in length. Later WGS84 was refined to have a semi-polar length of 6365752.314245 meters. This change results in calculation differences that are insignificant when considering the degree of accuracy (24 meters or ~ 80 feet) of the LEO_07 conversion routines, the accuracy of consumer-grade GPS receivers, and the precision associated with the tasks of spotting oil wells on a topographic map." (G. Gagnon, 2010, Specifying the geodetic datum in LEO_07, Kansas Geological Survey)

The REFERENCE CORNER determines the orientation of the spot with respect to the boundary lines of the section:

• SE corner: North of the South line and west of the East section line.
• SW corner: North of the South line and east of the West section line.
• NE corner: South of the North line and west of the East section line.
• NW corner: South of the North line and east of the West section line.

"Footages refer to the offset distance from the location spot to the boundary lines of the section. A footage corner is used to identify which boundary lines are used in calculating the footage offset. If the southeast corner is specified (default), then the program calculates the offset distance from the south and east lines. Likewise, the northwest corner would specify the offset with respect to the north and west boundary lines." (Gagnon, G.F., 2008, LEO Version 7.0 User Manual: Kansas Geological Survey, Open-file Report no. 2008-24, 20 p.).

Decimal Degrees for Kansas: The min-max range for Kansas is: 36.9930 to 40.0032 degrees latitude.

"Latitude (geographic) - Angle made by a perpendicular to a given point on the surface of a sphere or ellipsoid representing the Earth and the plane of the Equator (+ if the point is north of the Equator, - if it is south). One of the two common geographic coordinates of a point on the Earth." (Snyder and Voxland, 1989, An Album of Map Projections, U.S. Geological Survey Professional Paper 1453, page 3. Image from USGS Professional Paper 1395.)

Decimal Degrees for Kansas: The min-max range for Kansas is: -102.052 to -94.584 degrees longitude. Note: Western hemisphere longitudes are negative values.

"Longitude - Angle made by the plane of a meridian passing through a given point on the Earth's surface and the plane of the (prime) meridian passing through Greenwich, England, east or west to 180 degrees ( + if the point is east, - if it is west). One of the two common geographic coordinates of a point on the Earth." (Snyder and Voxland,1989,An Album of Map Projections, U.S, Geological Survey Professional Paper 1453, page 3. Image from USGS Professional Paper 1395.)

Acceptable LEO values range from 0 to 59. A "minute" is an angular measurement equal to 1/60th of a degree.

"The unit of surveying is the degree, defined as 1/360 of a circle. One degree (1°) equals 60 minutes, and 1 min equals 60 sec." (Ghilani, C. D. and Wolf, P. R. 2008, Elementary Surveying an Introduction to Geomatics: Pearson Prentice Hall, page 25.)

Acceptable LEO values range from 0.0000 to 59.9999. A "second" is an angular measurement equal to 1/60th of a minute. This field accepts seconds,decimal second values.

"The unit of surveying is the degree, defined as 1/360 of a circle. One degree (1°) equals 60 minutes, and 1 min equals 60 sec." (Ghilani, C. D. and Wolf, P. R. 2008, Elementary Surveying an Introduction to Geomatics: Pearson Prentice Hall, page 25.)

In Kansas, township values range from 1 to 35. All townships have a designated South direction.

"A township is a basic unit of land within the PLSS. Normally, townships are approximately 6 miles on a side. They are bounded on the north and south by township lines and on the east and west by range lines. Townships are designated by both a township and a range number."(Suchy, D.R., 2002, The public land survey system in Kansas: Kansas Geological Survey, Public Information Circular, no. 20, 4 pages.)

In Kansas, acceptable range values are 1 to 25 for the East range and 1 to 43 for the West range direction.

"Ranges are numbered east and west from the Sixth Principal Meridian--the east ranges are numbered from 1 to 25 to the Missouri border, and the west ranges from 1 to 43 to the Colorado border. Thus, for example, the first township east of the principal meridian and next to the Nebraska border is Township 1 South, Range 1 East (T. 1 S., R. 1 E.), whereas the one just west of the principal meridian is T. 1 S., R. 1 W. The township in the southeast corner of the state is T. 35 S., R. 25 E., and the one in the southwest corner of the state is T. 35 S., R. 43 W."(Suchy, D.R., 2002, The public land survey system in Kansas: Kansas Geological Survey, Public Information Circular, no. 20, 4 pages.)

Acceptable values are East and West.

"Ranges are numbered east and west from the Sixth Principal Meridian--the east ranges are numbered from 1 to 25 to the Missouri border, and the west ranges from 1 to 43 to the Colorado border." (Suchy, D.R., 2002, The public land survey system in Kansas: Kansas Geological Survey, Public Information Circular, no. 20, 4 pages.)

PLSS sections are numbered 1 through 36.

"Townships are divided into 36 sections of approximately 1 square mile or 640 acres each (fig. 2). Numbering of the sections begins in the northeast corner of the township and proceeds back and forth, east and west, until the last section, Section 36, is reached in the southeast corner of the township." (Suchy, D.R., 2002, The public land survey system in Kansas: Kansas Geological Survey, Public Information Circular, no. 20, 4 pages.)

Offset footages are positive whole numbers used to mark a spot within a section. The distance is measured in feet.

Footages refer to the offset distance from the location spot to the boundary lines of the section. A footage corner is used to identify which boundary lines are used in calculating the footage offset. If the southeast corner is specified (default), then the program calculates the offset distance from the south and east lines. Likewise the northwest corner would specify the offset with respect to the north and west boundary lines. (Gagnon, G.F., 2008,LEO Version 7.0 User Manual:Kansas Geological Survey, Open-file Report no. 2008-24, 20 p.)

The offset directions is determined by the REFERENCE CORNER.

• SE corner: North of the South line and west of the East section line.
• SW corner: North of the South line and east of the West section line.
• NE corner: South of the North line and west of the East section line.
• NW corner: South of the North line and east of the West section line.

Here the reference corner is SE and the spot directions are north of the South line and west of the East section line.(Gagnon, G.F., 2008,LEO Version 7.0 User Manual:Kansas Geological Survey, Open-file Report no. 2008-24, 20 p.)

When using a q-call spot, the user is associating (by closest approximation) the actual coordinate location to a fixed PLSS spot within the subdivision. There are nine possible “spots” within a subdivision

(Gagnon, G.F., 2008,LEO Version 7.0 User Manual:Kansas Geological Survey, Open-file Report no. 2008-24, 20 p.)

Q1 is the largest subdivision of a section. It represents an area approximately 160 acres in size. It is a quarter section.

“Quarter calls” are reported “spot first” then from smallest to largest subdivision. In this example the q-call location could be identified as being the center of the southwest quarter (smallest) of the southeast quarter of the southwest quarter (largest) of Section 7, Township 9S, Range 5E (sec. 7, T. 9S., R.5E.). The description is referred to as a “quarter-quarter-quarter call” or simply the “quarter call.”

(Gagnon, G.F., 2008,LEO Version 7.0 User Manual:Kansas Geological Survey, Open-file Report no. 2008-24, 20 p.)

Q2 represents an area approximately 40 acres in size. It is a quarter, quarter section.

“Quarter calls” are reported “spot first” then from smallest to largest subdivision. In this example the q-call location could be identified as being the center of the southwest quarter (smallest) of the southeast quarter of the southwest quarter (largest) of Section 7, Township 9S, Range 5E (sec. 7, T. 9S., R.5E.). The description is referred to as a “quarter-quarter-quarter call” or simply the “quarter call.”

(Gagnon, G.F., 2008,LEO Version 7.0 User Manual:Kansas Geological Survey, Open-file Report no. 2008-24, 20 p.)

Q3 represents an area approximately 10 acres in size.It is a quarter, quarter, quarter section.

“Quarter calls” are reported “spot first” then from smallest to largest subdivision. In this example the q-call location could be identified as being the center of the southwest quarter (smallest) of the southeast quarter of the southwest quarter (largest) of Section 7, Township 9S, Range 5E (sec. 7, T. 9S., R.5E.). The description is referred to as a “quarter-quarter-quarter call” or simply the “quarter call.”

(Gagnon, G.F., 2008,LEO Version 7.0 User Manual:Kansas Geological Survey, Open-file Report no. 2008-24, 20 p.)

Q4 is the smallest subdivision. It represents an area approximately 2.5 acres in size. It is a quarter, quarter, quarter, quarter section.

“Quarter calls” are reported “spot first” then from smallest to largest subdivision. In this example the q-call location could be identified as being the center of the southwest quarter (smallest) of the southeast quarter of the southwest quarter (largest) of Section 7, Township 9S, Range 5E (sec. 7, T. 9S., R.5E.). The description is referred to as a “quarter-quarter-quarter call” or simply the “quarter call.”

(Gagnon, G.F., 2008,LEO Version 7.0 User Manual:Kansas Geological Survey, Open-file Report no. 2008-24, 20 p.)

The UTM zones for Kansas are 13,14 and 15.

Image courtesy Kansas GIS Policy Board: http://newman.kgs.ku.edu/catalog/utm.html

"UTM zones are numbered easterly from 1 through 60 beginning at longitude 180 degrees West. The conterminous United States is covered from zone 10 (west coast) through zone 20 (east coast). The central meridian for each zone is assigned a false easting E of 500,000 m. A false northing N of zero is applied for the northern hemisphere of each zone, and 10 million m is assigned for the southern hemisphere to avoid negative Y coordinates." (Ghilani, C.D. and Wolf, P.R. 2008, Elementary Surveying: An Introduction to Geomatics (12th Edition), Prentice Hall, 960 p.)

UTM Northing: The north-south grid coordinate value specified in meters.

"Universal Transverse Mercator (UTM) grid - Military grid system based on the transverse Mercator projection, applied to maps of the Earth's surface extending from the Equator to 84 degrees North and 80 degrees South latitudes."(Thompson, M.M., 1979, Maps for America, Cartographic products of the U.S. Geological Survey and others: U.S. Geological Survey, 265 p.)

"UTM zones are numbered easterly from 1 through 60 beginning at longitude 180 degrees West. The conterminous United States is covered from zone 10 (west coast) through zone 20 (east coast). The central meridian for each zone is assigned a false easting E of 500,000 m. A false northing N of zero is applied for the northern hemisphere of each zone, and 10 million m is assigned for the southern hemisphere to avoid negative Y coordinates." (Ghilani, C.D. and Wolf, P.R. 2008, Elementary Surveying: An Introduction to Geomatics (12th Edition), Prentice Hall, 960 p.)

UTM Easting: The east-west grid coordinate value specified in meters.

"Universal Transverse Mercator (UTM) grid - Military grid system based on the transverse Mercator projection, applied to maps of the Earth's surface extending from the Equator to 84 degrees North and 80 degrees South latitudes."(Thompson, M.M., 1979, Maps for America, Cartographic products of the U.S. Geological Survey and others: U.S. Geological Survey, 265 p.)

"UTM zones are numbered easterly from 1 through 60 beginning at longitude 180 degrees West. The conterminous United States is covered from zone 10 (west coast) through zone 20 (east coast). The central meridian for each zone is assigned a false easting E of 500,000 m. A false northing N of zero is applied for the northern hemisphere of each zone, and 10 million m is assigned for the southern hemisphere to avoid negative Y coordinates." (Ghilani, C.D. and Wolf, P.R. 2008, Elementary Surveying: An Introduction to Geomatics (12th Edition), Prentice Hall, 960 p.)