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<< Click to Display Table of Contents >> General Tools - Generate Ground Control Point File (Workflow) |
  
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<< Click to Display Table of Contents >> General Tools - Generate Ground Control Point File (Workflow) |
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The Ground Control Point file (xml or shape format), which can be required as input for processes such as Orbital Correction, Geocoding, Interferogram Flattening, Persistent Scatterers and others, is generated.
Technical Note
Three different procedures are foreseen for the Ground Control Point creation:
1.A previously generated ENVI vector file (.evf) is used as input ("GCP Vector file"); additional information relevant to each point are manually entered only when required (refer to the Technical Note below). In order to generate the ENVI vector file (.evf) the following procedure is suggested:
•Load, in an ENVI "Image Window", the "Input file" image and select the "Create New Vector Layer" from the File menu.
•Select "Mode>Add New Vectors" (Vector Parameters panel).
•Select "Mode>Point" (Vector Parameters panel) and locate the GCPs as required. Click with the mouse right button within the ENVI "Image Window" and select "Accept as Individual Points".
It is worthwhile to mention that this procedure allows entering GCPs in either slant range (i.e. satellite view) geometry or cartographic projection; the user is prompted to specify the reference system once the .evf file is inputted.
2.The cartographic co-ordinates corresponding to the center of the ENVI "Zoom Window" are entered for each point. To locate the GCP in the image ("Reference file") just click with the left mouse button on the specific pixel within the ENVI "Zoom Window" and then click on the "GCP Position" button (SARscape panel) or, alternatively, insert manually "Row" and "Column" for each point in the SARscape panel.
3.An input "GPS file" in used; only the points falling within the "Reference file" coverage are used and converted in the output GCP file. No additional information relevant to each point can be entered. The use of this input file is not yet supported.
The use of the Ground Control Point file is foreseen (as possible or mandatory) in different processing steps; their number and position depend on the specific SARscape functionality where they are used (refer to the relevant Technical Note).
It must be noted that, if the Cartographic co-ordinates ("X", "Y", "Z") are entered - while SAR geometry co-ordinates ("Row" and "Column") are omitted - they will be converted in the geometry of the "Reference file" in order to get "Row" and "Column" position; moreover if the "Z" field is left blank, the GCP height will be retrieved from the input "DEM file" or from the "Cartographic System>Reference height" in case the DEM is missing.
When the "Displacement GCP file" is generated (PS or SBAS processing), the coordinates must be provided in cartographic units. In particular for the Refinement and Re-Flattening step, in the SBAS processing, the reference file must be the Super Master intensity image.
It is not possible to associate different Cartographic Systems to different points of the same GCP file. If the input "DEM file" is used, its Cartographic System will be the reference for all GCPs.
Here below we suggest the most suitable and easiest way (among the three described above) to generate the Ground Control Point file, depending on the specific processing where it is required.
1.As described above it foresees the collection of the GCPs in an ENVI vector file (.evf), which will be the input for the SARscape GCP file generation. The execution of the following processing steps - or the generation of the following input products - shall adopt this procedure:
| - | The "Coregistration file", which is optionally required when the coregistration is performed by manually locating the center of the coregistration windows (Coregistration; Coherence generation; Interferometric Workflows and Polarimetric SLC Coregistration). In these cases the fields relevant to the GCP position are not to be filled. |
| - | The "Displacement GCP file" in the Persistent Scatterers module. |
| - | The "Orbital GCP file" in the Dual Pair Interferometry module (see above conditions). |
| - | The "Orbital GCP file" ("Refinement and Re-flattening" in the Interferometry and ScanSAR Interferometry modules), when the GCP height ("Z") has to be automatically extracted from the available slant range DEM (_srdem) and when the GCPs are not affected by any measurable displacement. In these cases the fields relevant to the GCP position are not to be filled, unless additional points have to be added to those contained in the .evf file. |
| - | The GCP file, which is optionally required when the phase frequency removal is performed by manually locating the center of the windows used for its estimate, in the "Remove Residual Phase Frequency" interferometry tool. |
| 2. | As described above it foresees the localization of the GCPs in the image, directly through the SARscape panel by means of the "GCP Position" button or, alternatively, by manual insertion of "Row" and "Column" position. The execution of the following processing steps - or the generation of the following input products - shall adopt this procedure: |
| - | The "Geometry GCP file" in the Persistent Scatterers module. In this case only the "Row", "Column", "Y", "X" and "Z" fields relevant to the GCP position have to be filled. |
| - | The "GCP file" which is a mandatory input in the Manual Orbital Correction and an optional input in the Geocoding and Radiometric Calibration; the Map to Slant image transformation; Map to Slant Shape Transformation; the Coregistration; the Interferometric Workflows; the Interferogram Flattening and "Refinement and Re-flattening" (Interferometry and ScanSAR Interferometry modules), the "Synthetic Phase Generation" (Polarimetric Interferometry and ScanSAR Interferometry modules) with an input Digital Elevation Model. In these cases only the "Row", "Column", "Y", "X" and "Z"fields relevant to the GCP position have to be filled. |
| - | The "Orbital GCP file" in the Dual Pair Interferometry module (see above conditions). |
| - | Pulse Repetition Frequency Correction process. |
| - | The "Orbital GCP file" ("Refinement and Re-flattening" in the Interferometry and ScanSAR Interferometry modules), when the height ("Z") of the Ground Control Points is not automatically extracted from a slant range DEM (_srdem); this happens for instance in the case the user wants to manually enter a GCP height or when the Ground Control Points are affected by a measurable displacement. In these cases some or all of the fields relevant to the GCP positions must be filled (refer to the "Input Parameter" description below). |
| 3. | This procedure can be adopted only in case the GPS files are in SINEX format. When this procedure is adopted it is not possible to add or modify the input GCP list coming from the GPS. It is usually adopted for: |
| - | The "Orbital GCP file" ("Refinement and Re-flattening" in the Interferometry and ScanSAR Interferometry modules), when the information (position, displacement history and velocity, etc.) relevant to each GCP comes from a GPS survey. |
Vx,y,z
These parameters are entered only when the GCP file has to be used for interferometric related processing and in particular for displacement mapping. The GCPs come from measurements, typically collected during ground truth campaigns, which must be entered here as velocity units (mm/year). In case the collected information is available in metric units (e.g. millimeters or centimeters) instead of velocity, it must be transformed by considering the time interval between master and slave acquisition.
As an example a co-seismic displacement of 50 cm for an interferometric pair acquired at 35 days distance will correspond to a velocity of around 5214 mm/year.
It is important to note that these velocity fields can be associated only to GCPs whose location is provided in cartographic co-ordinates (i.e. "X", "Y" and optionally "Z"); vice versa these fields are not taken into account when the GCPs location is entered as file co-ordinates (i.e. "Column", "Row"). If this parameter is not provided the GCP displacement velocity is set to zero (stable point).
Input file
It is used for:
| - | Determining the multilooking factors of the image where the GCPs have been located. Note that it is possible to use the same set of GCP onto the same "Input File" with different multilooking factors. |
| - | Determining the reference time of the GCP measurement (position and velocity). |
Input File(s)
Output file
Output file name of the Ground Control Point file. This file is mandatory.
Input file
The SAR image which the GCP (or GPS points) refers to. This file is mandatory.
GPS file (not yet supported)
A GPS file (.snx), containing the Ground Control Point(s) cartographic position and - possibly - the displacement history. This file is optional.
DEM file
It is used to retrieve the height of those GCPs whose height ("Z") has not been entered. This file is optional.
GCP Vector file
A previously generated vector file (.evf), containing the Ground Control Point(s) position. This file is optional.
Input Parameter(s)
Column
File co-ordinate, in the "Input file" geometry, of the selected pixel (x direction).
Row
File co-ordinate, in the "Input file" geometry, of the selected pixel (y direction).
Label
A text identifier can be entered for each point. If the GCP file has to be used afterward in the Pulse Repetition Frequency functionality, this label is mandatory and it must be: "reference_gcp".
Collection Date
The date (DD-MMM-YYYY; e.g. 12-JUL-2010) which the GCP position, and its displacement velocity, refers to. This parameter is optional, in case it is omitted the acquisition date of the "Input file" is used instead.
X (Easting)
East-West cartographic co-ordinate of the selected pixel.
Y (Northing)
North-South cartographic co-ordinate of the selected pixel.
Z (Height)
Elevation in meters (above the ellipsoid) of the selected pixel.
Vx
Displacement velocity (mm/year) in East-West direction. This parameter is normally (not necessarily) entered only when the GCP file has to be used in the "Refinement and Re-flattening" processing step aimed at Displacement Mapping.
Vy
Displacement velocity (mm/year) in North-South direction. This parameter is normally (not necessarily) entered only when the GCP file has to be used in the "Refinement and Re-flattening" processing step aimed at Displacement Mapping.
Vz
Displacement velocity (mm/year) in vertical direction. This parameter is normally (not necessarily) entered only when the GCP file has to be used in the "Refinement and Re-flattening" processing step aimed at Displacement Mapping.
In case that the Digital Elevation Model is not used, following parameters are compulsory:
State
Definition of the country or general projection systems.
Projection
Definition of the projection system of the selected State. In case that a general projection system is selected in State, the Projection is automatically set.
Ellipsoid
Definition of the ellipsoid. This is chosen according to the selected State and Projection.
Hemisphere
Definition of the hemisphere. This is chosen according to the selected State and Projection.
Zone
Definition of the zone. This is chosen according to the selected State and Projection.
Datum Shift Parameters
Definition of the datum shift parameters. These are chosen according to the selected State and Projection.
Reference Height
In case that the Digital Elevation Model is not used, a constant ellipsoidal height must be provided. Default Reference Height is 0.
Cartographic Parameters
The reference parameters for some projection systems (e.g. Stereographic, Polar Stereographic, Gnomonic, Mercator, Miller, Albers, etc.) can be set.
Output(s)
Root name of the output file.
.xml
Ground Control Point file to use as input in any processing step where it is required.
.shp
Shape file of the Ground Control Points.
Details specific to the Units of Measure and Nomenclature of the output products can be found in the Data Format section.
General Functions
Start
Start of the processing.
Store Batch
The processing step is stored in the batch list. The Batch Browser button allows to load the batch processing list.
Cancel
The window will be closed.
Help
Specific help document section.
Specific Function(s)
GCP Position
The file co-ordinates (Row and Column) corresponding to the cursor location are displayed.
Insert GCP
The file co-ordinates corresponding to the Ground Control Point position are stored.
Update GCP
If the parameters of a selected Ground Control Point are modified in the underneath table, the changes are saved by clicking this button.
Delete GCP
The selected Ground Control Point is deleted.
References
None.