Javad TRIUMPH-LS User Manual page 114

direction type.
Mag Geo and Mag Grid
Mag Geo and Mag Grid both apply the magnetic declination
at P1 to the Geo and Grid direction types using the time-
dependent World Magnetic Model (WMM) 2010-2015.
J-Field will continually and automatically be updated to that
model in accordance with all official NGA releases.
Distances
Distances can also be viewed in one of four types by
toggling the distance button.
Geo
Š
Grid
Š
Ground
Š
Slant
Š
Geo distance
Geo distance type is the distance between two points as
measured along the ellipsoid.
Grid distance
Grid distance is the 2D (North and East) Cartesian inverse of
two grid coordinates. Depending on the projection design
and the location of the inverse points (P1 and P2) within the
projection, the Grid distance can vary significantly from the
Geo (ellipsoid) distance.
Ground distance
The Ground distance type, generally the most important
type to surveyors, is the distance between two points as
measured along the design surface, such that the elevated
reference surface can be thought of as passing through the
average ellipsoid height of P1 and P2. Note that this distance
type is related to the ellipsoid height of P1 and P2. If other
points of interest vary significantly in ellipsoid height from
the height of these two points, the scale of the ground
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distances between those points will also vary as a result.
Geometric closures using these distances may suffer as a
result, as an inverse at an average ellipsoid height of 1000
meters will not correlate well to an inverse at an average
ellipsoid height of 500 meters by a factor of approximately
1ppm per 6.4 meters (21 feet) of change in ellipsoid height.
In this example, an ellipsoid height difference of 500 meters
affects the distance by 78ppm.
Slant Distances
Slant distance type is the direct geocentric Cartesian (ECEF/
XYZ) inverse of the coordinates of P1 and P2, sometimes
referred to as Mark-Mark. Slant distance between two
points is not affected by the current coordinate system.
Coordinate Systems
z
P1
P2
Slant distances
Mark to Mark
x
y
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