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PHIDIAS

The complete solution for close range photogrammetric applications

· 3D ­ Models of buildings · As-built-models of industrial facilities · Deformation analysis · Stereo Aerial Photogrammetry · Data collection for GIS · Reconstruction of accidents and crime scenes · Photo rectification · Visualization

PHIDIAS - Digital Photogrammetry with MicroStation

PHIDIAS is a powerful digital photogrammetric system which is used for any kind of image measurement. PHIDIAS is integrated into MicroStation and well suited for 3-dimensional documentation of complex buildings and facilities.

Digital images from different sources are the base for many different measurement tasks in architecture, achaeology, preservation of monuments, industry and criminalistics.

PHIDIAS

Photogrammetry with MicroStation

PHIDIAS is based on the widely available CAD-System MicroStation (Bentley Systems) and can be obtained in three different versions: · PHIDIAS-MS is the full version of this product series. The user is able to manage most demanding measuring tasks by means of processing very large image blocks. The professional measuring of complex objects is thus made possible by applying the most modern techniques. · PHIDIAS-DUO only uses two images respectively for the measuring of objects (image pairs). Its application only makes sense if smaller objects or parts of objects have to be evaluated and the integration of the results into a higher context is not necessary. · With PHIDIAS-MONO you can evaluate two-dimensional objects like plane fassades. Perspective distorted images can be rectified and several orthophotos can be assembled to produce image maps.

PHIDIAS running in MicroStation

PHIDIAS is the latest development in the field of photogrammetric evaluation systems and runs as MDLApplication under the CADsystem MicroStation. The data generated by PHIDIAS will be directly stored in MicroStation design files and processed there. You can use PHIDIAS with all Micro-Station Eingineering configurations like GeoGraphics, TriForma or PlantSpace without problems.

PHIDIAS processes digital images which are loaded into the views of MicroStation in order to evaluate them. The user carries out the geometric evaluation by means of the digital images loaded on the screen without being restricted by the operating possibilities of MicroStation. This means, for example that the command "place smartline" can be activated and executed in MicroStation as normal. PHIDIAS intercepts the placement of points in views if an orientated image is loaded and makes sure that the element is being drawn considering the photogrammetric orientation parameters.

PHIDIAS

Photogrammetry with MicroStation

The obtained evaluation result is superimposed as a drawing on the photographs. (Superimposition, see fig.) The embedding of the photogrammetric element PHIDIAS in MicroStation presents many advantages for the user of which a selection is listed below: · The extensive selection of commands in MicroStation for the generation and processing of plans is at the user's disposal for the photogrammetric evaluation. The efficient generation of drawings is for example supported by basic functions such as copy, move, mirror etc. · As a result of the superimposition of the drawing on the images, the user can see at a glance whether the evaluation on the screen is correct and complete. Thus, the user is able to correct many errors already during the recording, which occurs especially when measuring three-dimensional, complex objects. · MicroStation supports special tools such as "dimension driven cells". With this, geometric elements with the same basic structure (e.g. views) can be filed in their own libraries and pasted during the photogrammetric evaluation adjusting the respective dimension. The measuring tasks are thus mainly reduced to the positioning of the objects. · In the process of the evaluation, surfaces and volume elements can be generated directly. In comparison to the pure wire frame model, this model offers multiple visualization possibilities in connection with the powerful rendering-tools from MicroStation. · As far as surface -and volume elements have been generated during evaluation, sectional drawings can afterwards be generated automatically from the measured objects. The cutting plane can be placed anywhere in space. · In order to link specific alphanumerical information with the geometric drawing data, the user can refer to external data bases by means of an interface. Thus, specific data such as the condition and kind of material etc. can be recorded simultaneously with the photogrammetric evaluation and then stored in a relational data base. Other MDL-Applications like GeoGraphics, TriForma or PlantSpace can also be used together with PHIDIAS.

The user who is familiar with MicroStation usually does not have great difficulties to carry out photogrammetric evaluations with PHIDIAS. The handling is quite similar to the work with MicroStation.

PHIDIAS

Photogrammetry with MicroStation

Photogrammetric evaluation with PHIDIAS - The digitized images are loaded into the views of MicroStation. The user "draws" on the digital images and knows the exact the state of the evaluation. Via the mouse the user can open a magnifier where the scale factor can be arbitrarily adjusted and thus any particular image section can be enlarged. As a result of this the precise location of points is possible.

The orientation module in PHIDIAS

A prerequisite for the photogrammetric evaluation is a mathematical reconstruction of the situation of the camera positions (image orientation). Depending on the version, PHIDIAS is provided with the corresponding algorithms and support in order to execute this task. The orientation module from PHIDIAS aims especially at user friendliness. Statistical operations and plausibility checks are built into every step of the orientation process in order to check the data material for gross errors. By doing so, failures during orientation can be avoided in most cases. The user does not have to leave MicroStation for the image orientation. The operation referring to the dialogue is adapted to the known "look and feel" MicroStation's principle (dialogue boxes, choice in list boxes via mouse click) so users without a profound knowledge of photogrammetry also get on okay. The integrated orientation program carries out a complete bundle adjustment of all observations and is provided with all necessary modules for the supply of approximate values.

PHIDIAS

Photogrammetry with MicroStation

The orientation can be carried out automatically in many cases without manual interference. The necessary approximate values are defined with the utmost possible reliability. All gross errors are automatically discovered and are eliminated. Thus, the only tasks for the user are the start of the calculation of the orientation and the transfer of the results of the evaluation. Apart from that, every single step of the orientation (relative orientation, absolute orientation) can be executed under interactive control of the user. By doing so, partial results such as accuracy or the dimension of gross errors are indicated. Single steps also give the possibility to run the respective part of the calculation automatically. More special features of the orientation module are listed below: · the number of images to be processed simultaneously is only limited by the size of the RAM · qualified calculation of values of accuracy (standard deviations of the orientation results) · estimation of the accurary of the evaluation by means of the orientation data · processing of non-photogrammetric observations (lines, coordinates, etc.) · definition of additional requirements · simultaneous camera calibration with different mathematical methods of distortion correction · orientation of images without control points (orientation in free net adjustment) · estimation of variance components for the qualified estimation of the obtained accuracy of measurement · high-precision definition of points by means of bundle adjustment

A comprehensive manual describes the operation of the PHIDIAS-orientation module and gives further general background information on the photogrammetric image orientation.

PHIDIAS

Photogrammetry with MicroStation

Image capturing

PHIDIAS works with digitized measurement photographs. There are several possibilities to collect digital image data: · A digital camera with a CCD-sensor (i.e. Kodak DCS or Nikon D100) is used for the photographs. · The negatives are directly digitized with a film-scanner. · The negatives are transfered to a Photo-CD (Kodak) in special labs. · The photographs are enlarged on positives and then digitized with a a flatbed-scanner · Analog images on video-tapes are digitized later on. (frame grabber) These possibilities mainly differ in cost, the quick availability of the data and the geometrical scanner resolution. All these factors have an influence on the attainable accuracy. Depending on the measuring task either a digital or analog camera should be preferred. The following summary shows the main differences between digital and analog photography. Digital photography Advantages: · · · Easy image capturing High geometric stability Instant control of image quality Analog photography with negative film Advantages: · · · · · High resolution Better reproduction of colors Larger density range Longer battery life Nearly unlimited image storage

Disadvantages: · · · · High costs for professional SLR camera systems Compact cameras with built in lenses have very high distortion and the calibration is not stable Limited image memory and high consumption of power Long exposure times produce noise

Disadvantages: · · · Film deformation reduces geometric accuracy Control of exposure and image quality after film development Images need to be scanned

Specific functions for evaluation

Measuring functions The photogrammetric measuring operation with PHIDIAS is in principle based on the monoscopic localisation of homologous points. In contrast to the stereoscopic measuring method this means that the points that describe the object ­geometry have to be measured sequentially in order to be able to locate them exactly (principle of homology). A high accuracy of measurement can be achieved because of the possibility to measure object points in up to eight images at the same time.

PHIDIAS

Photogrammetry with MicroStation

Furthermore, special measuring functions are implemented in PHIDIAS which allow a semistereoscopic measurement technique in a broad sense. I.e. you do not have to adhere to the principle of homology so strictly during the photogrammetric measurement of certain geometrical structures. The user for example does not necessarily have to measure the homo-logous initial -and final points of the edge of an object; it is sufficient if the measuring points are somewhere on the line. Thus, the user is able to record parts of the objects which can only be seen partially on the photographs. The above mentioned measuring functions mainly apply to the following components: · · · · · Straight edges of an object Circular arcs and circles Cylindrical surfaces (e.g. pipes) Spherical surfaces or volumes Cones and truncated cones

These measuring functions specific to PHIDIAS can be applied particularly well for the measuring of industrial plants because the above mentioned geometrical structures are very common there. The concrete measuring process will be described here by means of the following example: recording of cylindrical pipes ("place pipes"). The evaluation of photogrammetric images which aims at the measurement of pipings can cause difficulty in locating distinct points, which define the geometry of the pipe in a suitable manner. It is difficult to define the axis of the pipe accurately because of the more or less large diameter. Without the previous attachment of targets it is difficult to identify identical points on the objects. However, single points - tangent points - can be defined on the lateral contours but without correspondence between the images. The cylindrical sections of tubular surfaces would be recorded as follows with PHIDIAS: In every image the user has to locate four points on the contours (see figure). I.e. two points are measured on each side. Every cylindrical element is to be defined by four tangents points respectively - hereby the user does not have to observe a homologous coordination between the images. After these measurements are carried out with two or more images, the location and dimension of the cylindrical surface will be calculated. Finally, the results you obtain are the middle axis and the diameter of the pipe section, which is shown as a three-dimensional volume-model in the drawing. The above mentioned example gives an impression on how the special measuring functions can be used during the evaluation with PHIDIAS. The user is thus able to record the cylindrical sections of a piping system conveniently. Further special features aim at the effective measuring of straight or circular structures in a similar manner; the user does not have to adhere too strictly to the homology between images despite the monoscopic measuring method.

PHIDIAS

Intersection with arbitrary surfaces

Photogrammetry with MicroStation

When the geometry of the surface is known, you can select this element as a cutting element. Measurements on this surface can then be made monoscopic and you can easily place windows into cylindrical walls or spherical roofs and place pipe components on a cylindrical tank. Homogenizing functions So-called homogenizing functions complement the above mentioned special measuring functions. Homogenizing is the graphical processing from the photogrammetric evaluation with a limited accuracy to the exact drawing. It includes for example the creation of constraints like orthogonality, parallelity and planarity between adjacent line elements, which are not directly presented as a result because each measuring process can only be carried out with limited accuracy. PHIDIAS is provided with corresponding functions for the interactive homogenizing of the photogrammetric evaluation. All actions take place in the CAD-environment of MicroStation with the user controlling the manipulations in the usual manner. In respect to operation and controlling, the homogenizing functions work similar to other drawing functions in MicroStation, e.g. copy, move etc. The photogrammetric survey of a piping system will be given here again as an example. The relevant homogenizing function includes the following possibilities:

1. The sections of the bends/curves which form the connection between the straight sections can be automatically constructed by the user marking all axis of the cylindrical parts interactively. The sections of the bend/curve are then completed, in order to get a pipage without any gaps. 2. Additionaly, fixed diameters can be set through the input of specific values. 3. The user can determine whether exact orthogonalities are to be produced between adjacent pipe sections in the homogenizing process. Here, tolerances can be specified according to which PHIDIAS decides automatically whether a right angle exists or whether the given angle should remain unchanged. 4. Furthermore, there is the option to arrange the vertices of a piping section into a plane. This plane can be a specific coordinate-plane of the local coordinate system or any plane in space. This option aligns the corresponding piping sections by means of homgenizing exactly in one plane, which is, in practice, justified due to the constructional conditions. 5. Besides, the user can determine whether the initial -and final points of a piping section are to be fixed or movable. This is important for the further processing of connected sections.

The above mentioned possibilities of controlling within the scope of the homogenizing of pipes are to give an impression of the homogenizing tools in PHIDIAS. Similar functions, which are aimed at the respective purpose of homogenizing, are available for other geometrical structures.

PHIDIAS

Stereo Measurement In combination with a mirror stereoscope or LCD shutter glasses you can evaluate stereo images three-dimensional. You can use stereo measurement in single screen or multi screen configurations. Two views are used for the left and right stereo image which will be transformed automatically so that no parallaxes - even with convergent directions - will be seen.

Photogrammetry with MicroStation

The three-dimensional cursor is controlled by the mouse and keyboard so that you can evaluate even elements with curved surfaces. All MicroStation tools and commands can be used for evaluation. Of course the superimposed drawing will also bee seen stereo.

Automatic target detection

The database for the work with PHIDIAS are the digital images. The advantage of digital images over analog images is that many tasks can be automated. Measurement speed and accuracy can be improved significantly. Automatic target measurement: The automatic target measurement can be used for several special tasks like high accuracy deformation analysis. You only need to prepare the object with suitable targets. Normally the target signs are black circles on white background or white on black background. The size of the signs should be about 10 pixels in the digital image. Signs consisting of 4 quarter circles can also be used.

Sophisticated and fast algorithms ar implemented in PHIDIAS which detect the sign with very high accuracy and reliability. The center of the sign is the center of the contour-ellipse or ­ circle which separates the dark shape from the bright background. The accuracy that can be achieved is about 1/5 to 1/20 of the pixel size. The user only has to move the cursor near the sign or ­ if approximate values for the orientation parameters and object coordinates exist ­ he can start an automatic interactive or non-interactive measurement of all known points.

PHIDIAS

Photogrammetry with MicroStation

Automatic edge measurement: Especially in close range applications like architectural photogrammetry or as-built documentaion of plants most object structures consist of straight or curved edges wich appear as edges with high contrast in the digital image. PHIDIAS can detect these edges automatically. Similar algorithms as for the target detection are used. The position of the edge can be calculated with subpixel accuracy. The edge measurements can be used for the placement of line elements and they also be used to improve the bundle adjustment.

Image recification

Often a realistic representation drawn to scale, including all details is more useful than a line drawing or a simplified 3D-model. PHIDIAS has several tools for image rectification, orthophoto production and for an easy and fast creation of photorealistic virutal models. · Image recification by projective transformation A rectification of plane surfaces is possible, when 4 control points are known. The calculation of the orientation parameters is not necessary. Also the camera parameters are not required. Only with the use of lenses with a strong distortion the camera calibration parameters should be known. If more than 4 control points in the image are measured, the redundancy can be used for a smooth distribution of the residuals. Large facades are divided into smaller sections and several rectified images can be combined in a single view to print to scale or output as single georeferenced digital image.

PHIDIAS

·

Photogrammetry with MicroStation

Rectification of plane surfaces In images which are already oriented, a polygonal surface can be selected and rectified to an othogonal projection. The transformed image bitmap can be attached as a material definition to the surface. From Objects that consist of few plane surfaces, realistic virtual 3D-models can be built very easy. Unwrapping Arbitrary curved walls, e.g. with an outline formed by a circular arc or even by splines, can be unwrapped.

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Orthophoto by projection to 3D-pointcloud 3D-pointclouds like they are captured with laserscanners, are used to map images to amorpous surfaces. The pointcloud defines the surface to which any oriented image can be mapped to produce a orthogonal projection.

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Orthophoto by projection to 3D-surfaces or ­volumes A surface defined by a pointcloud with low resoltion is sometimes ambiguous. As alternative the drawing elements like surfaces and volumes can be used for orthophoto projection.

Combined evaluation with laserscanner data

3D-pointclouds, which are captured e.g. with a laserscanner, can be used in combination with photogrammetric images. On the condition that the coordinate system in both cases is identical, the point cloud replaces the missing third dimension in the images. By this means a 3D single image measurement is possible.

PHIDIAS

Photogrammetry with MicroStation

The combination of both procedures leads to several advantages: · · · · Small details of the scanned object beyond the scanner resolution, which are normally lost, are visible in the high resolution photogrammetric image.Wrong interpretation can be avoided. Time-consuming stereo measurement can be replaced with mono image measurement. Single photogrammetric images can be oriented separate without taking notice of the angle between the photographs. Orthophotos can be produced directly. The third dimension is already known.

Accuracy

General information on accuracy usually does not make sense for photogrammetric closerange applications for which PHIDIAS is designed for, because the obtained accuracy of measurement depends on many factors which differ from project to project. Basically we have to distinguish between the accuracy of evaluation when graphically defining the object contours on the screen and the accuracy which can be obtained by means of measuring single points followed by bundle adjustment in the orientation module of PHIDIAS. The latter is much higher.

PHIDIAS

Photogrammetry with MicroStation

Evalution accuracy The most important factors are the geometrical resolution during the digitizing process and the scale of the images. With the help of these two parameters the following rough calculation can be made for the photogrammetric evaluation accuracy: An object which has to be measured photogrammetrically has a dimension of about 20 m. The images are digitized with a resolution of 3000 pixels per line. If we simplify by saying that a point in those images can be localized to this accuracy, this corresponds to a value at the object of about 20 m / 3000, i.e. approximately 0.7 cm. As experience shows this value has to be multiplied by a factor of 2-3, because further influences such as film processing, errors in localization and camera positions affect the accuracy. Especially the camera positions have a great influence on the attainable accuracy. For a precise estimation of the evalution accuracy PHIDIAS can carry out corresponding simulation calculations, which also take the image configuration into account. Accuracy for single point measurements in combination with bundle adjustment PHIDIAS can also carry out high-precision measuring tasks (e.g. deformation measurements) making the relative measurement accuracy of single object points - referring to the dimension of the object - from 1:25.000 to 1:30.000 possible. Before taking the photographs the object points have to be marked with suitable target marks and a sufficient number of images with a good geometrical constellation have to be supplied. After measuring the single points, the three-dimensional coordinates of the object points are calculated by means of the PHIDIAS-bundle adjustment. With the help of statistically-derived values (standard deviations etc.) the actual obtained accuracy can then be judged in an objective manner.

System requirements

PHIDIAS runs in MicroStation 95, MicroStation SE and MicroStation /J under Windows 95 / 98 / NT 2000. There are no special requirements to the hardware. You only need enough RAM to store the digital images. The image size is not limited, nevertheless enough RAM will reduce hard disk access and speed up image display. A system with multiple screens is recommended when you work with many images, views, dialogs and toolbars.

PHOCAD Ingenieurgesellschaft mbH Jülicher Straße 336 b 52070 Aachen Germany Tel.: +49 241 96092-44 Fax: +49 241 96092-46 Email: [email protected] http://www.phocad.de

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