All Topo Map V7 Coordinate, Projection and Datum Support

Quick Links Coordinate Styles:
  [ DMS ] [ DM ] [ D.d ] [ UTM ] [ State Plane ] [ PLS ] [ Free Form ]
Quick Links Concepts:
  [ Datum ] [ Datum conversion NADCON/ Molodensky ] [ Distance ] [ Relative Coordinate ]

All Topo Maps V7 supports all reasonable coordinate styles, datum and projections.

Latitude / Longitude DMS.s

You can enter coordinates with dialog based 'helper' boxes. To find a Latitude/Longitude, Degree, Minute, decimal Second style coordinate location, or enter it as a waypoint, press the 'Find Location' button: then choose the 'DMS' tab:

Once a coordinate is entered, it is automatically converted to all of the other supported coordinate styles. You can customize each of the style separately, and the coordinates can be highlighted and copied to the Windows clipboard.

Latitude / Longitude DM.m

To find a Latitude/Longitude, Degree, decimal Minute style coordinate location choose the 'DM.m' tab:

Latitude / Longitude D.d

To find a Latitude/Longitude, decimal Degree coordinate location choose the 'D.d' tab:

UTM (Universal Transverse Mercator) Coordinates

To find a UTM Universal Transverse Mercator coordinate location choose the 'UTM' tab:

All Topo fully supports alternate UTM datum and units (like feet and survey feet)!

MGRS Coordinates

To find a Latitude/Longitude, decimal Degree coordinate location choose the 'D.d' tab:

State Plane Coordinates

To find a state plane coordinate location choose the 'State Plane' tab:

Public Land Survey (PLSS) Coordinates

The optional 'PLS Tool' for the state map set is required to enable PLS coordinates.

To find a location using Township Range Section choose the 'PLS' tab:

All Topo Maps will aliquot by section, half section, quarter section and quarter-quarter section. You can choose a coordinate as aliquot center, or one of the four corners (NW, NE, SE, SW).

Free Form Coordinate Styles

In addition to using the coordinate entry boxes on the 'Find Location' tool, you can enter coordinates in plain text format:

The All Topo engine accepts a variety of coordinate entry formats:

• Latitude / Longitude : DMS.s, DM.m or D.d

• Universal Transverse Mercator (UTM)

• Military Grid Reference System (MGRS)

• State Plane Coordinates (SPC)

• Public Land Survey (PLS)

• Relative coordinates (@...)

Once you enter a coordinate, the All Topo engine will dependably 'figure-out' what your intentions are.

The engine uses a complex series of rules to interpret coordinates:

If you enter a number proceeded by the word 'Zone' or the letter 'Z', All Topo assumes you are specifying a UTM coordinate and looks for two additional numbers, with optional units and datum, to be interpreted as easting and northing:

1,689,810 Ft E, 14,940,739 Ft N Z13 WGS72
515,129.86 M E, 4,554,132.93 M N Z13

If no Zone or 'Z' is found, and 2, 4 or 6 unique numbers are found; All Topo assumes you are entering a Latitude / Longitude coordinate. Degree-minute-seconds (DMS.s), degree-minute (DM.m) and degree (D.d) entries are supported:

41.140,278° N, 104.819,722° W 2
41° 8.416,7' N, 104° 49.183,3' W
41° 8' 25.000" N, 104° 49' 11.000" W

Any coordinate that is comprised of an integer number, followed by three letters, followed by 2, 4, 6, 8 or 10 number characters is interpreted as a MGRS coordinate.

A coordinate that contains a valid State Plane Zone code (like 'WYE') and two numbers is assumed to be a State Plane Coordinate:

595,562.0 Ft E 172,734.5 Ft N WYE

A coordinate that contains 'SEC' (section) and 'PM' (principal meridian), 'SEC' and 'PRIN' or 'SEC' and a valid (princiapal meridian code) is assumed to be a Public Land Survey coordinate (the PLS option is required):

3,024 W 3,253 N from SE Sec 31 T 14N R 66W PM6
3,024 W 3,253 N from SE Sec 31 T 14N R 66W Sixth

Relative Coordinates

In addition to fully-qualified coordinates, it is possible to enter relative coordinates that specify a new coordinate at an offset from a previous coordinate.

Relative coordinates always begin with the '@' (ampersand) and specify a distance and bearing from the previous base location. Relative coordinates may be stacked, however there must be a fully qualified coordinate before the first relative coordinate. To specify a relative coordinate you can enter:

Radial or Cartesian offsets with Rotation. Example: up ¼ mile, right ½ mile, plus 2.4 rotation

'@ X=0.5 miles Y=-0.25 miles R=2.4'

Offset Direction: Bearing and North Reference. Example: 

'@ L=1.25 miles R=34.5' Compass bearing

or

'@ N55.5E 1.25 miles' Survey Angle

All Topo accepts True, UTM Grid, State Plane Grid and Magnetic North references; and Great Circle (Lat/Lon), UTM Grid and State Plane Grid projections. All Topo assumes True North Reference and Great Circle (Lat/Lon) projections unless explicitly overridden.

Datum

The following table summarizes datum supported by All Topo Maps V7:

When entering coordinates using the "Find Location" tool you indicate the coordinate datum using a dropdown box:

When entering a coordinate in an annotation source file, you just append the datum code to the location:

42 23 24 N 109 26 24 W NAD83

What is Datum?

For maps that show a very large area of the earth, it is sufficient to model the earth as a sphere. However for detailed coordinate use, the earth must be treated as an oblate ellipsoid also called an oblate spheroid (think of a basketball flattened at the North and South poles).

    flattening ratio = w/(w-h)

Two geometric constants are used to define the ellipsoid, the semi-major (w) and semi-minor (h) radii. Typically we express these constants as the semi-major axis and the flattening ratio.

While the flattening ratio is only about 1 part in 300, it becomes an important part of equations modeling the earth and for calculating geographic coordinates.

The choice of the reference ellipsoid has been a major concern of geodesists since the early 18th century. In the 18th century Isaac Newton postulated that the Earth should be slightly flattened at the poles. The French Academy of Sciences in 1835 sent expeditions to Peru and Lapland to measure meridians at widely separated latitudes. Following this confirmation of Newton's hypothesis, there were 26 determinations of the Earth's dimensions between 1799 and 1951.

The Bessel ellipsoid of 1841 was used in the United States from 1844 until 1886 when the Clarke 1866 ellipsoid was adopted and used for almost every map covering North America until the present time.

In 1909 John Hayford derived a reference ellipsoid from U.S. Coast and Geodetic Survey measurements specific to the United States. This ellipsoid was adopted by the International Union of Geodesy and Geophysics (IUGG) in 1924 and is called the 'International Ellipsoid'. It is currently used in many parts of the world (most maps of the Hawaiian Islands use the International ellipsoid.) Because the Clarke 1866 ellipsoid was already so prevalent in North America, it prevailed over the International Ellipsoid.With the introduction of satellite data, it has been possible to more accurately determine the ellipsoid. The U.S. military produced the World Geodetic System of 1966 and 1972 (WGS66 and WGS72). In 1980 the IUGG adopted the Geodetic Reference System 1980 (GRS80) from which the National Geodetic Survey has based the North American Datum 1983 (NAD83) replacement for the North American 1927 Datum (NAD27). U.S. Military agencies also developed the World Geodetic System 1984 (WGS84), based upon GRS80, that is the native basis for the GPS measured coordinate systems. The following list summarizes common datum used in the United States:

The U.S.G.S. is systematically converting many maps to NAD83 datum. As a result, the coordinates of most points in the United States change slightly from old NAD27 maps to new NAD83 maps. A location's coordinate can change over 1,000 feet in some areas between NAD27 and NAD83!

Converting Between Datum

The conversion from NAD27 to NAD83 is not a simple function. In fact, the conversion cannot accurately be expressed as a function.

There are two common methods for converting between NAD27 and NAD83/WGS84 datum for coordinates in the United States and its territories:

• NADCON interpolations

• Molodensky approximations

NADCON interpolations use a grid of points defining the difference between NAD27 and NAD83 coordinates. The grid points are held in a matched series of .LAS and .LOS files generated by the National Geodetic Survey (NGS), based upon very accurate terrestrial, gravity and GPS measurements. Approximately 2 megabytes of data are required to hold complete interpolation data for the United States.

Molodensky approximations use a set of spherical constants to convert between NAD27 and NAD83 datum. Molodensky constants are computed to minimize errors over a small local area and as a result there are unique constants for Mexico, Continental United States (NAD27CONUS), Canada (NAD27CANADA) and Alaska (NAD27ALASKA).

NADCON interpolations are almost perfect conversions, but require large data files to cover the United States. NADCON style conversions are not available for worldwide conversions.

The Molodensky equations (with a set of transformation coefficients optimized for the local area) will convert from one datum to another and very closely match NADCON style approximations. Molodensky Errors are typically less than 20 meters over the area of intended coefficient use.

All consumer (and most professional) GPS devices convert from WGS84 datum to NAD27 / NAD83 datum using Molodensky approximations. Molodensky constants are available for worldwide conversion and do not require the use of large data files. However, it is very important to choose Molodensky constants that are optimized for the local area.

When you enter a coordinate in All Topo Maps it is assumed to be a NAD27 (NADCON) data coordinate. To enter a NAD83 coordinate add 'NAD83' after the coordinate value. To indicate that the coordinate should be treated as a CONUS Molodensky approximation, add NAD27CONUS after the coordinate.

Distance Units

All Topo Maps accepts the following units for any horizontal distance measurement:

If no units are specified, All Topo assumes that distances are expressed in Meters.

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