PhoTopoL - a digital photogrametric station

PhoTopoL is a powerful, low cost software application which produces precise maps from aerial photography. Developed by TopoL Software Ltd. Prague, Czech Republic. PhoTopoL performs orthorectification to remove geometric distortions and the effects of terrain relief displacement in aerial photos. PhoTopoL also produces Digital Elevation Models by correlation of stereo pair. The software's stereo plotting capabilities provide ideal tool for editing and updating digital maps and DEMs. More over in PhoTopoL full TopoL GIS system is included and all data are captured directly into this GIS. PhoTopoL is running on a standard PC's under MS Windows 9X or MS Windows NT.

Functions

Orientation

The system calculates first the interior orientation and then external orientation. This external orientation of a photo or a stereo pair can be calculated as standard relative orientation plus spatial transformation of a model into ground coordinates or by Bundle Adjustment method. To calculate the orientation of one image or a stereo pair, one has to have

• parameters of camera (calibration protocol)
  focus length
  coordinates of fiducial marks
• altitude of flight (approximate), not important
  usually printed on the photograph
• at least 4 full (X, Y, Z coordinates) control points

The system has got also extension to external Aerotriangulation module AeroSys.

Orthorectification

Orthorectification removes the effects of relief displacement and imaging geometry from aerial photos. Orthorectified imagery has broad applications in the photogrammetric and GIS fields. Rectification can by done pixel by pixel or by triangles where the size of triangles is user defined. Grey values or colours can be calculated by Nearest Neighbourhood method or by Bilinear Interpolation. Orthorectified images can be joined into mosaics.

Correlation to Generate Digital Elevation Models

By using a digital stereo correlation algorithm the system extracts elevation values from stereo pair. The raster DEM can be created in the TopoL system for orthorectification. With extra module Atlas one can create flexible elevation models for orthorectification or generation of contour lines. This module includes a powerful engine for adding and editing breaklines and obligatory lines, displaying of hypsometry or slops etc.

Stereo Viewing and Editing

Two monitors are used and the active superimposition of all vectors is performed on both of them. This makes it possible to detect any changes between the new photograph and the old map. View and edit in stereo using floating or fixed cursor offer possibility to register X,Y,Z coordinates. During the registration object attributes can be assigned immediately. A zoom function enables zoom-out and zoom-in on any area of image. Zoom-in even up to subpixel size. Stereo viewing is provided through StereoGraphics Corp. Crystal Eyes or Z-Screen. Flickerless viewing is achieved by splitting the high monitor vertical scan rate. The viewing system presents the left and right perspective views to the corresponding eye in synchronization with the Eyewear.

It is two monitor system so any two graphics card has to be installed in the computer. DirectX vs. 6.0 is used to display the data on the stereo screen.

Hand wheels, (e.g. Microsoft EasyMouse) or tablet can be used as 3D pointing device.

Modularity

The system is fully modular. It is possible to have the separate module only for orthorectification, correlation, DEM creation or stereo editing. The most important part of PhoTopoL is the calculation of image or stereo pair orientation. Before somebody can run orthorectification, correlation or stereo editing they have to calculate the orientation first. It is useless to have orthorectification or correlation module without this module.

If somebody wants to run only orthorectification they can have only the module for orthorectification. In this case they have to have also the tool to create or import the DEM. It means they have to have at least the functionality of TopoL DEM for creation of DEM.

The function of a digital stereo correlation is used if somebody does not have the DEM or does not have the contour lines or points with elevation from desired area. In that case they can use this algorithm to find out the points with elevation automatically. Then these points can be used as an input for the mentioned DEM creation.

It is possible to have a special module only for stereo editing, this can be connected to TopoL Reduced or higher.

If somebody wants to run the stereo editing or the correlation they have to run the epipolar transformation first. This can be done on separate computer especially if they want to have more stations only for editing.

We recommend these minimal modules

1. Orthorectification - module for the orientation calculation, the orthorectification, module for DEM import and creation.
2. Correlation - module for the orientation calculation, the epipolar transformation, the correlation.
3. Stereo editing - the station to prepare the data has to have, module for the orientation calculation, the epipolar transformation, the stereo editing station has to have stereo editing connected to TopoL Reduced or higher.

These are the standard sets

1. PhoTopoL Orthophoto - TopoL for Windows DEM, the orientation calculation, the orthorectification.
2. PhoTopoL Generation - TopoL for Windows DEM, the orientation calculation, the orthorectification, the correlation, Atlas.
3. PhoTopoL Stereo - TopoL for Windows DEM, the orientation calculation, the orthorectification, the correlation, Atlas, the stereo editing.

As a part of the software package PhoTopoL the module Atlas can be delivered. It is a system for interactive processing of general surface in the 3D space. The standard module which can work with up to 60.000 points includes these functions

• Generation of DEM,
• Interactive surface editor
• Contour lines calculation
• Export of DEM and contour lines to DXF

It is possible to have also higher version of Atlas with Perspective views and the module for the generalisation of the DEM by the excluding points which has got no importance for DEM. It is possible to have also MEGA version which runs up to 4 millions points in one DEM.

Software