Coordinate Converter - Transforming Coordinates

Coordinate Conversion - The Concept

 

Coordinate conversion between two different datum is traditionally performed using the seven-parameter conversion. The seven parameters consist of three for the translation vector, three for the rotation matrix and one for the scale factor:

On the left side, the information regarding the input coordinates that are to be converted is presented. On the right side, the output (resulting) coordinates after conversion are shown. To perform the conversion, the “Convert” button in the middle needs to be pressed. The conversion procedure is described in the following sections.

Step-by-step procedure for coordinate conversion - Direct Mode

 

Suppose that we wish to convert the coordinates of a site, named SiteA. The coordinates of this site were obtained by GPS, using a reference base whose coordinates are in ITRF2008 datum referenced at epoch June 1st 2005. The datum we wish to convert these coordinates to is NAD83-NSRS. These are the main steps needed to perform the conversion:

 

1. Choose the datum of the input coordinates in the Datum combo box:

· Use None: Choose this option if you do not need any temporal adjustment of the coordinates either because no information regarding site movement is available or because the input and output epochs are the same. Please note that if this option is chosen, the output epoch is automatically adjusted to be the same as the input’s and that it cannot be edited. Also, the input and output velocities boxes are disabled.

· Use input: Choose this option if the site velocity information you have is for the input datum. In this example, if site velocity information is available in ITRF2008, we may choose this option. The velocity input boxes are then enabled and may be filled in either in cartesian (xyz) format or local geographic (North-East-Up) format. The velocity may also be automatically estimated via interpolation (see Coordinate Converter – Using Station Velocity).

· Use output: Choose this option if the site velocity information you have is for the output datum. In this example, if there is no velocity information available for ITRF2008 but you have it for NAD83, you may choose this option and fill in the output velocity boxes.

It displays the Coordinate Converter.

The next step is then to perform the datum conversion:

Note that the epoch of the output coordinates is the same as the epoch of the input coordinates.

 

Now, suppose that the coordinates were taken with GPS at a marker, for which official coordinates are available but are in NAD83 datum, epoch 2002. We cannot directly compare both coordinates because they have different epochs and the site may have moved between these two epochs. We are referring here to movement due to various tectonic and geological processes which affect all points on Earth. To quantify this movement, the concept of “Site velocity” is used. This velocity is a 3D vector, whose components are usually given in millimeters per year (mm/y). Then, if we wish to convert coordinates from one epoch to another, information about the site velocity is needed. The seven-parameter conversion is not enough.

If a velocity vector () is either directly provided or estimated through interpolation, we can then temporally translate a coordinate between two epochs. For our example, we have:

Where,

 : temporal difference

Please take into account that this method has some limitations. First, the velocity is not always constant but can change from year to year. Second, if an interpolation method is used, this method cannot always accurately model the complex movements of the tectonic plates, especially near faults. For more information about the interpolation methods used in Coordinate Converter, please refer to the Coordinate Converter – Using Station Velocity article.

The User Interface

 

Start OnPOZ Tools in Windows Main Menu under Effigis. Then select Coordinate Conveter.

2. Choose the epoch of the coordinates. Here we have three format options:

a. Year Month Day (yyyy mm dd)  format : 2005  06 01

b. Year + Day of Year (yyyy doy ) format : 2005 152

c. Decimal year : 2005.41370

Note that once we input an epoch with a specific format, the epoch is automatically converted when we change once again the format.

Please note that only numerical values, the minus sign and the decimal point are accepted in the input coordinates boxes.

4. Choose if you wish to apply site velocity information for temporal adjustment of the coordinates.

Step-by-step procedure for coordinate conversion in Batch Mode

 

If you wish to convert a list of coordinates, use the batch mode. To do so, choose the fourth option in the format combo box:

If a CSV file format is chosen for the input coordinate, the same format is automatically set for the output coordinate.

The seven conversion parameters are usually valid for a specific date but they are often given along with a set of temporal translation and rotation parameters so that the conversion parameters may be extrapolated to another date. Nevertheless, after conversion, the output coordinates always have the same epoch as the input coordinates.

 

For example, let us consider that we have some input coordinates in the WGS84 datum at a 2013 epoch. We wish to convert them to the NAD83 datum.  Lets suppose that the conversion parameters to convert from WGS84 to NAD83 are given for a 1997 epoch.

 

The first step is to temporally translate the conversion parameters from the 1997 epoch to the 2013 epoch:

: Output (converted) coordinates vector

 : Input coordinate vector

: Translation vector

: Rotation matrix

 : Scale factor (scalar)

Where,

3. Input the 3D coordinates. Here we still have three possible input formats:

a. 3D Cartesian , i.e. XYZ coordinates

b. Geographic coordinates (latitude, longitude, height) in Degrees, Minutes, Seconds (DMS)

c. Geographic coordinates (latitude, longitude, height) in decimal degrees

There is a fourth option, EZSurv CSV file, which will be explained in the following section.

Three options are available:

If you wish to export the output coordinates to another program, press the “Copy to Clipboard” button.

5. Select the output datum. In our example, it is NAD83-NSRS.

6. Select the epoch of the output coordinates, only if site velocity information is available.

7. Select the format for the output coordinates (same formats available as input coordinates).

8. Press the convert button.

Then, follow the same steps as for the direct mode case.  Please note that for the velocity estimation, we have also three choices:

· Interpolation, where the automatic velocity estimator will be used (see Coordinate Converter - Using Station velocity).

· User defined (x,y,z) : The same velocity (xyz format), taken from the velocity input boxes, will be used for all sites.

· User defined (n,e,u) : The same velocity (North-East-Up format), taken from the velocity input boxes, will be used for all sites.

The input file must be a csv (comma separated values) file containing either Cartesian or geographic coordinates. There are also two main restrictions to the format of this csv file:

1.      It must contain a one-line header containing the identifiers of each column. The file may contain many columns but the program needs at least the following identifiers :

Lat, Lon, EllHgt

and/or

Cart_X,Cart_Y,Cart_Z

These identifiers correspond to the ones used when exporting position files with EZSurv. If these identifiers are not present in the header, the conversion will fail. The position of these identifiers is not important as long as the data is in the same column.

Examples:

GPSWeek,GPSSecond,Cart_X,Cart_Y,Cart_Z

 

GPSWeek,GPSSecond,Lat,Lon,EllHgt,StdDevLat,StdDevLon,StdDevHgt,Solution,NDOP,EDOP,VDOP,TDOP,HDOP,PDOP,GDOP,GPS,GLONASS,Galileo,Cart_X,Cart_Y,Cart_Z

Note that, optionally, the file may contain comment lines, beginning with // or /* (C/C++ style), which will be ignored.

2.      Each value of a line of data must have the same position as determined by the header.

Geographic coordinates must be given in decimal degrees.

Example:

GPSWeek,GPSSecond,Cart_X,Cart_Y,Cart_Z

590,237716,1266031.459,-4292007.591,4529727.668

Article: 000115

Related Products: EZField, EZSurv, EZTag CE, OnPOZ Tools

Last Update: 2017-04-06 19:49:00

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