I often get questions about using the Modified Northing, and the Modified
Easting which are created in Leica Geo Office after computing and applying the
average combined factor. Below are three questions which illustrate the concerns
and a response which covers all three questions relating to modified coordinates
(ground coordinates) and state plane coordinates (grid coordinates) and
geographic positions, as well as the relationship of TPS to these coordinate
systems and to GPS:

Question 1:
 
Why does the computation associated with "raising coordinates to the surface,"
shift them so far horizontally from their true geographic (grid) location?

Question 2:
 
How should data be adjusted so as to raise coordinates to the surface while
still maintaining their true geographic location (i.e. centered geographically
over their grid location)? It would be great if the coordinates would be scaled
to their surface location and also be centered geographically over the grid
coordinate locations, so that this data can be used for surveying, engineering
and other applications that can be spatially referenced data is available (i.e.
orthophotos, shape files, etc.).
 
Question 3:
 
How do we easily re-adjust data once both types are collected in the same file?
If I collect data with GPS, then set up the total station on one GPS point and
backsight another, these coordinates are then written to the raw data file and
essentially restrict these points along with any TPS sideshots collected from
the setup from being adjusted once imported in LGO, without going to each point
and copying and pasting triplets and further updating backsight azimuths
associated with the setup orientation. This current method is too time consuming
on large projects and allows the introduction of human error if some setups are
not updated manually.
 
 
RESPONSE

Modified coordinates do not relate to any geographic position. They are simply
the state plane coordinates (which can be related to geographic positon by the
state plane coordinate system) scaled by the average combined scale factor so
that the distances and directions between them will be gound distances and grid
azimuths. There is no point held fixed as a point to scale relative to.
Generally, some value is subtracted from the northing and easting values so that
they do not get confused with the state plane coordinates they are derived from.

Modified coordinates are not intended to be utilized directly. They are only
half of the solution. In order to use these points with GPS (and TPS that will
be mixed with GPS) we need to create a coordinate system that will relate the
WGS 84 positions of the points with the ground coordinate positions. This can be
done using either a one-step or a two-step transformation depending on the size
of the project. A guide to using a 2-step transformation can be found at this
link http://askellis.net/index.php?option=com_content&task=view&id=2&Itemid=7.
Once this project ground coordinate system has been created, it can be used to
properly relate the ground coordinates to the WGS 84 positions.

To efficently combine both TPS and GPS measurements, the TPS instrument should
use a valid coordinate system, and should be set to compute the GeoPPM
corrections automatically. The TPS always measures on the local side of the
transformation using local coordinates. When the active coordinate system is the
project ground system its measurements will fit the ground distances. But when
the instrument is working in the state plane coordinate system, the distance
measurements will be appropriately scaled to grid based on the position of the
measurement within the project.The appropriate coordinate system needs to be
selected for the coordinates being used, both in the TPS and GPS instruments.
This automatic scaling in the TPS requires a valid coordinate system. It will
not work if the coordinate system is set to <None> or WGS 1984.

It is not possible to work with state plane and ground coordinates in the same
coordinate system. Coordinate systems allow us to use TPS and GPS together,
relating a local coordinate system (either assumed ground or state plane) to WGS
84. If we have both a ground coordinate system and a state plane coordinate
system that are using the same WGS 84 reference, we can use the appropriate
coordinate system to work in either ground or state plane with the TPS and GPS,
but not at the same time. We need to select the appropriate coordinate system
for the coordinates we are using at the moment. When importing TPS data
into LGO, the correct coordinate system should be active in both the job on the
instrument and the project in LGO.

In LGO, if the points have WGS 84 positions, simply changing the coordinate
system will provide either ground coordinates, or state plane coordinates. Once
the ground coordinate system has been created, it is recommended to turn off the
computing of the modified northings and eastings as they have no relevance when
the coordinate system is set to ground and they are not needed when the
coordinate system is set to state plane. Leaving the modified coordinates active
is just confusing.

Changing a coordinate system in a job that has only local points, will have no
effect on the local coordinates. A coordinate system relates local coordinates
to WGS 84 coordinates. It does not relate local coordinates to local
coordinates. Importing TPS points from a job without a coordinate system (the
coordinate system is None or WGS 84) will require the use of Shift, Rotate and
Scale to move them into coincidence with existing points. If the TPS points are
imported from a job using the project ground coordinate system into an LGO
project with the the state plane coordinate system attached, the imported points
can be aligned with the existing points by using the Exchange Coordinate System
function. This only works if the TPS job had a valid coordinate system, and the
two coordinate systems must relate to the same WGS 84 positions.

SUMMARY

The use of the modified coordinates created in LGO requires the creation of a
coordinate system if these coordinates are to be used in conjunction with GPS.
This coordinate system should be used in the TPS instrument so that the measured
points can be related to the GPS when they are imported into the LGO project.
When combining TPS and GPS always use a valid coordinate system. This coordinate
system can be either the grid or the ground, but both must relate to the same
WGS 84 positions. It is recommended to configure the TPS instrument to compute the GeomPPM automatically. That way the measured distances will be properly scaled depending on the coordinate system used.

Here is a link to Leica's Webinars, including one on Grid & Ground -
http://www.leica-geosystems.com/us/en/lgs_65879.htm?cid=9455.