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Grid / Ground Parameters
To set Grid / Ground parameters:
- In Parameters, select which set of parameters to enter: Scale factor, Average Job Height (more...) or Origin Point (more...).
- In Origin Pt and Scale Factor modes directly enter the Combined Scale Factor to be used in the transformation. In any Parameters mode resulting combined scale factor will be displayed at the bottom of the dialog for reference purposes.
- In Avg Job Ht mode enter the Map Scale Factor from Ellipsoid to Grid to be used in the transformation.
- In Scale Factor mode you can select the Direction of the scale factor (either "Ground to Grid" or "Grid to Ground"), depending on which information you have. In Origin Pt mode, the "Ground to Grid" direction is always selected, and in Avg Job Ht mode, the "Ellipsoid to Grid" direction is always selected.
- Enter the Average Job Height to be used in the transformation.
-
Enter the angle value of Azimuth Rotation / Bearing Rotation directly if it is known.
Note: You can set direction as bearings or azimuths in the Display dialog.
- Click
to calculate the rotation from bearings / azimuths on Grid and Ground. More... Azimuths/Bearings in turn can be calculated from points in the job. More...
- In Scale Factor or Avg Job Ht modes enter Offsets of the origin along the North and East axes. These offsets are from the Geodetic North -> Ground North.
- In Origin Pt mode select an origin point from either map or the list.
- In Origin Pt mode enter the coordinates for the ground point.
Learn more... about how to perform Grid/Ground transformation.
Compute Rotation
To calculate rotation from azimuth / bearing on Ground and Grid:
- You can either manually enter the Grid azimuth / bearing and Ground azimuth / bearing or click the Compute button to calculate azimuth / bearing using points in the job that define the needed directions.
- After entering the Grid azimuth / bearing and Ground azimuth / bearing values, the Rotation field displays the calculated azimuth rotation or bearing rotation value.
- Click
to close the window.
Compute Azimuth / Bearing
To compute the azimuth / bearing by two points that define the direction:
- In From, enter the name of the initial point or select the point from the map
or the list
.
- In To, enter the name of the terminal point or select the point from the map
or the list
.
- Enter an angle to Add to Azimuth / Add to Bearingif required.
- View the computed Azimuth/Bearing.
Grid to Ground Transformation
MAGNET Field supports two methods for setting the relation between Grid and Ground coordinate systems. One method performs scaling and rotation relative to some point of the job. The other method performs scaling and rotation relative to the origin of the Grid coordinate system.
To get started with transformation:
- Click the Configure icon
and then the Coordinate System icon
.
- Select the desired projection through which the link will be found with some Ground coordinate system. Only after selecting the projection the option of Grid to Ground transformation will become available.
- To activate the Grid to Ground (or vice versa) transformation select the Use Grid/Ground check box.
- Press
to select a desired transformation method (from "Origin Pt", "Avg Job Ht" and "Scale Factor") and set the corresponding parameters for this method.
Find out more:
Creating Ground Projection Relative to a Point
Creating Ground Projection Relative to the Origin of Grid System
Parameters of transformation from the Grid to Ground coordinates can be used in GNSS stake of road points. For example, to stake a point at the distance of 100 meters from some point, you have to create a ground coordinate system, which will work as a base coordinate system in GNSS stake calculation. To do this, select the corresponding Grid and enter the average height of a desired portion of the road. The created ground coordinate system enables you to stake points at special distances in the grid system.
Creating Ground Projection Relative to a Point
If you know the coordinates of a point in both coordinate systems (Grid and Ground) and also rotation of these systems, then to find the relation between Grid to Ground coordinate systems, select "Origin Pt" from the Parameters list on the Grid/Ground Parameters screen.
This method calculates an offset vector in the horizontal plane between coordinates of a point (called the origin point) in the grid and ground coordinate systems and computes the ground coordinates from the grid coordinates using this offset. If a rotation angle is present between these coordinate systems, MAGNET Field can rotate a grid or ground coordinate system relative to this point. Also, MAGNET Field takes into account a scale factor between these coordinate systems. See the plot below:
After determining the relation between both coordinate systems, MAGNET Field will calculate ground coordinates from the grid coordinate system and vice versa.
Look at the example of using this method below:
The current MAGNET Field job contains two sets of measured points:
- One network of points measured by a GR-3 receiver from a reference station in Grid coordinate system, for example, SPC83-Ohio(North).
- Another network of points measured by a Topcon Total Station (GTS-220) in the Ground coordinate system with the arbitrary choice of zero BS azimuth.
These networks have:
- A common point - the point CP2 of the GNSS network and point CP2_TS of the TS network. This point is the origin point for our transformation.
- A common line - the line between CP2 and CP6 points of the GNSS network, and the line between CP2_TS and CP6_TS of the TS network. The azimuths of this line in different networks will be used for calculation of the rotation angle between both coordinate systems.
See Map View for the different networks:
To perform Grid to Ground transformation for these networks:
- Click
, select the desired Grid system (SPC83-Ohio(North)) in the Projection field of the Coordinate System screen.
- Check the Use Grid / Ground box to use this transformation, and press
to open the Grid/Ground Parameters screen.
- Select "Origin Pt" in the Parameters field to activate this method.
- Select the origin point in the Grid coordinate system (CP2 point) from the list.
- Remove the coordinates of the selected point in the fields Northing and Easting which MAGNET Field automatically writes in the Grid coordinate system.
- Type in the values of the Ground coordinates of the origin point.
- Clicking
in the Grid/Ground Parameters screen:
- Calculates a combined scale factor for this point.
- Combines the GNSS point with the TS point. After that, in the Ground coordinate system, the GNSS point will have horizontal coordinates of the corresponding TS point, and, in the Grid coordinate system, the TS point will have horizontal coordinates of the corresponding GNSS point.
- Performs grid to ground transformation (and vice versa) taking into account the scale factor.
The Map displays the networks either in the Ground or Grid coordinate system:
To take into account the rotation between these two networks in this transformation:
- As we see from Map View for the different networks, the grid azimuth is set by direction of the line CP2 - CP6, and the ground azimuth is set by direction of the same line in ground coordinate system (the line CP2_TS - CP6_TS). MAGNET Field calculates the corresponding azimuth when you select the start and end points of the line.
- Click
in the Azimuth Rotation field. The Compute Rotation screen allows calculating Grid and Ground azimuth to obtain rotation angle between two coordinate systems (see Grid to Ground Transformation Without Rotation).
- Click the Compute button in the Ground line. In the Compute Azimuth screen, select the corresponding points (that define the common line in the Ground coordinate system) from the list.
- Click the Compute button in the Grid line. In the Compute Azimuth screen, select the corresponding points (that define the common line in the Grid coordinate system) from the list.
- The final rotation angle between two coordinate systems for our example is displayed in the Rotation field. Click
to save this calculation.
- The Grid/Ground Parameters screen contains all needed values to calculate the relation between the Grid and Ground coordinate systems. Click
in this screen.
- The Grid to Ground transformation is successfully performed. Below is the a plot that displays the error of transformation taking into account the scale factor and rotation for the given example:
Creating Ground Projection Relative to the Origin of Grid System
If you know the value of the scale factor between grid and ground coordinate systems or the average height of the network, then, in the Grid/Ground Parameters screen, you can select "Scale Factor" or "Avg Job Ht".
This method calculates plane ground coordinates by scaling, offsetting, and rotating grid coordinates. Find below the graphic explanation of transformation by scaling only, by rotating only, and by shifting only:
by scaling |
|
by rotation |
|
by shifting |
|
To find a relation between Grid and Ground coordinate systems:
- If the scale factor is known, select "Scale factor" in the Grid/Ground Parameters screen, type in a desired value, and select the correct direction (Grid to Ground or Ground to Grid) for this value.
- If the average height of the network is known, select "Avg Job Ht" in the Grid/Ground Parameters screen and type in the desired value of the height in the Avg Job Ht field.
The Scale Factor will be automatically calculated from: Scale_Factor = (1+ Avg Job Ht/Mean_Earth_Radius), where Mean_Earth_Radius = 6371000.0 m
- In both cases you can enter the rotation and offsets:
- If the angle of rotation is known, type in this value into the Azimuth Rotation field. MAGNET Field performs rotation of the Ground relative to the origin of the Grid coordinate system.
- To shift the Ground system from the selected Grid, type in the desired plane offsets on the Northing Offset and Easting Offset fields.
The final coordinates in the Ground will be calculated from: Nground=Ngrid+Northing_Offset and Eground=Egrid+Easting_Offset.
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