---
title: g.gui.image2target
description: wxGUI GCP Manager
index: topic_GUI|GUI
---

## DESCRIPTION

The **GCP Manager** is a *[wxGUI](wxGUI.md)* extension which allows the
user to create, edit, and manage Ground Control Points. It is available
from the menu "File \| Manage Ground Control Points".

The **GCP Manager** provides an interactive graphical interface to
manage and analyze Ground Control Points. A backup copy of the initial
POINTS file is always maintained and updated only on request (Save GCPs
to POINTS file). This guarantees that accidental changes are not
permanent and can be undone by reloading the Ground Control Points.

The GCP Manager must be started in the target project, not in the source
project (project used to be called location).

The GCP Manager is structured into three panels:

- The topmost panel shows a list of Ground Control Points. Tools to
  manipulate and analyze GCPs are provided in the toolbar. This panel
  can be moved out of the GCP manager window by either dragging with the
  caption or by clicking on the pin button on the right in the caption.
  This panel can also be placed below the map displays by dragging.
- The two panels in the lower part are used for map and GCP display, the
  left pane showing a map from the source project and the right pane
  showing a reference map from the target project. Numbered Ground
  Control Points are shown on both map displays.

### Components of the GCP Manager

![GCP Manager](wxGUI_gcp_frame.jpg)
  
#### Toolbars

Two toolbars are provided with the GCP Manager, one for managing the map
displays and one for managing the GCP list.

#### List of ground control points

The list of Ground Control Points can be sorted by clicking on a column
header. Clicking on a column header will sort the GCPs ascending, a
second click on the same column will sort the GCPs descending. Overall
RMS error and individual RMS errors of all points are often improved if
the GCP with the highest RMS error is adjusted. Individual coordinates
can be edited by double-clicking on a row.

The first column holds a checkbox and displays the point number. A GCP
is only used for RMS error calculation and georectification if its
checkbox on the left is checked. Uncheck to deactivate a GCP (mark as
unused GCP).

#### Two panels for map display

The left panel is used to display a map from the source project, the
right panel to display a map from the target project. Zooming in and out
is always possible with the mouse wheel and done for each map canvas
separately.

GCPs are displayed in different colors, depending on whether a GCP has a
high RMS error, is currently unused or is currently selected.
Optionally, currently unused GCPs are not shown on the map display.

#### Statusbar

At the bottom of the GCP Manager is a statusbar providing several
functions. The default is set to *Go to GCP No.* (see also below).
Typing a number or using the up/down arrows will center the maps on the
given GCP, useful with a high zoom.

#### GCP Map Display Toolbar

![icon](icons/show.png)  *Display map*:
Displays maps for source and target canvas and re-renders any layers
that have changed since the last time the display was updated.

![icon](icons/layer-redraw.png)  *Re-render map*:
Re-renders both source and target canvas regardless of whether they have
changed or not.

![icon](icons/erase.png)  *Erase display*:
Erases both source and target canvas to a white background.

![icon](icons/gcp-create.png)  *Define GCP (Ground Control Points)*:
On left mouse click, coordinates are defined for the currently selected
GCP.

![icon](icons/pan.png)  *Pan*:
Interactive selection of a new center of view in the active display
monitor. Drag the pan cursor while pressing the left mouse button to
pan. Alternatively left-click on the new center. Panning changes the
location of the region displayed but not the size of the area displayed
or the resolution.

![icon](icons/zoom-in.png)  *Zoom in*:
Interactive zooming with the mouse in the active map canvas (source or
target). Drawing a box or just a left click with the mouse and zoom-in
cursor causes the display to zoom in so that the area defined by the box
fills the display. The map resolution is not changed. Clicking with the
zoom-in cursor causes the display to zoom in by 30%, centered on the
point where the mouse is clicked. Zooming changes the display region
extents (both size and location of area displayed).

![icon](icons/zoom-out.png)  *Zoom out*:
Interactive zooming with the mouse in the active map canvas (source or
target). Drawing a box or just a left click with the mouse and zoom-out
cursor causes the display to zoom out so that the area displayed shrinks
to fill the area defined by the box. The map resolution is not changed.
Clicking with the zoom-out cursor causes the display to zoom out by 30%,
centered on the point where the mouse is clicked. Zooming changes the
display region extents (both size and location of area displayed).

![icon](icons/zoom-more.png)  *Adjust display zoom*:
Source and target display are adjusted by using the current GCPs for
coordinate transformation:  
  
- *Adjust source display to target display*:
The extents of the source display are adjusted to the current extents of
the target display.

- *Adjust target display to source display*:
The extents of the source display are adjusted to the current extents of
the target display.

*Set active map canvas*:
Sets the currently active map canvas (source or target). Click to set
active map canvas for *Return to previous zoom* or *Zoom to extent of
currently displayed map*. Alternatively, move the mouse over the map
canvas to be used as active canvas.

![icon](icons/zoom-last.png)  *Return to previous zoom*:
Returns to the previous zoom extent. Up to 10 levels of zoom back are
maintained.

![icon](icons/zoom-extent.png)  *Zoom to extent of currently displayed map*:
Zoom to the extent of the currently displayed map in the active map
canvas (source or target).

![icon](icons/settings.png)  *Settings*:
Shows a settings dialog for GCP management and display:  
  
- *Symbology*:
Settings for map and GCP display:  
  
  - *Highlight highest RMS error only*:
Only the GCP with the highest RMS error will be displayed in a different
colour, both in the list of GCPs and the GCP Map Display.

  - *Factor for RMS error threshold = M + SD \* factor:*
All GCPs with an RMS error larger than mean RMS + RMS standard deviation
\* this factor will be displayed in a different colour, both in the list
of GCPs and the GCP Map Display. As a rule of thumb, GCPs with an RMS
error larger than *M + SD \* 2* are most probably wrong. GCPs with an
RMS error larger than *M + SD \* 1* are worth closer inspection. This
option is only available if *Highlight highest RMS error only* is
unchecked.

  - *Color*:
Set the color for GCPs on the GCP Map Display.

  - *Color for high RMS error*:
Set the color for GCPs with a high RMS error on the GCP Map Display.

  - *Color for selected GCP*:
Set the color for the currently selected GCP on the GCP Map Display.

  - *Show unused GCPs*:
If unchecked, unused GCPs will not be shown on the GCP Map Display.

  - *Color for unused GCPs*:
Set the color for unused GCPs on the GCP Map Display.

  - *Symbol size*:
Set the symbol size for GCPs on the GCP Map Display.

  - *Line width*:
Set the line width for GCPs on the GCP Map Display.

  - *Select source map to display*:
Select a source map for the left pane of the GCP Map Display.

  - *Select target map to display*:
Select a target map for the right pane of the GCP Map Display.

- *Rectification*:
Settings for georectification:  
  
  - *Select rectification method*:
Set the polynomial order for georectification. This order will also be
used for RMS error calculation.

  - *Clip to computational region in target project*:
Clip raster maps to the current computational region in the target
project when georectifying.

  - *Extension for output maps*:
Change the extension for output map names when doing the actual
georectification.

![icon](icons/help.png)  *Show Help*:
Show help page for the GCP Manager.

![icon](icons/quit.png)  *Quit*:
Quit the GCP Manager.

#### Toolbar for the GCP list

![icon](icons/gcp-save.png)  *Save GCPs to POINTS file*:
The current list of GCPs is saved to the imagery group's POINTS file and
to a backup copy.

![icon](icons/gcp-add.png)  *Add new GCP*:
Adds a new Ground Control Point to the list and selects it for editing.

![icon](icons/gcp-delete.png)  *Delete selected GCP*:
Deletes the currently selected GCP from the list.

![icon](icons/gcp-remove.png)  *Clear selected GCP*:
Resets all coordinates of the currently selected GCP to 0 (zero).

![icon](icons/reload.png)  *Reload GCPs from POINTS file*:
Reloads GCPs from the imagery group's POINTS file.

![icon](icons/gcp-rms.png)  *Recalculate RMS error*:
Recalculates forward and backward RMS error for all GCP marked for use
(activated checkbox in first row).

![icon](icons/georectify.png)  *Georectify*:
Uses *[i.rectify](i.rectify.md)* to georectify all images in the source
imagery group.

#### GCP Map Display Statusbar

The GCP map display statusbar is similar to the statusbar in the regular
GRASS map display with two differences, *Go to* has been replaced
with *Go to GCP No.* and *Projection* has been replaced with *RMS
error*.

If *Go to GCP No.* is selected, a GCP number can be given in the left
side of the statusbar and the source and target map canvas will be
centered on the given GCP. Clicking on the map canvas will update
coordinates for this GCP.

If *RMS error* is selected, the overall forward and backward RMS error
is displayed.

## SEE ALSO

*[wxGUI](wxGUI.md), [wxGUI components](wxGUI.components.md)*

*[i.rectify](i.rectify.md), [m.transform](m.transform.md),
[v.rectify](v.rectify.md)*

See also [video
tutorials](https://grasswiki.osgeo.org/wiki/WxGUI/Video_tutorials#Georectifier)
on GRASS Wiki.

## AUTHORS

Markus Metz  
  
*Based on the Georectifier (GRASS 6.4.0)* by Michael Barton  
Martin Landa, Czech Technical University in Prague, Czech Republic

## SOURCE CODE

Available at: [g.gui.image2target source code](https://github.com/OSGeo/grass/tree/main/gui/wxpython/image2target)
([history](https://github.com/OSGeo/grass/commits/main/gui/wxpython/image2target))  
Accessed: Tuesday May 12 01:23:38 2026
