The Charter of Cultural Heritage of Puglia
The thirty-year activity of the architectural photogrammetry laboratory of the Polytechnic of Bari was based on three basic principles:
Of great importance are the surveys carried out by the Photogrammetric Unit of the Bari Traffic Police in the areas affected by the 1980 earthquake.
With unclick on "Restitution" the restitutor panel opens, with the couple of the selected frames, which allows the stereoscopic vision and the use of the same.
The open archive
Forty years later, the open photogrammetric archive of Architectural Heritage seems to have achieved the objective set by the C.I.P.A. (Comité International de Photogrammétrie Architecturale) in its first meeting (Paris, 18 and 19 June 1970), that is the establishment of an international archive of Photogrammetric Surveys of Architectural Heritage, duplicated and appropriately located in two safe centers .
The main difficulties that hindered the creation of this archive were essentially:
The renderer online photogrammetric
The photogrametry allows, through a couple of photographs, taken with a particular technique, to know the spatial coordinates of each point observable on both frames. There is talk, more precisely, of stereophotogrammetry, and should make the distinction between:
The novelty lies in the easy accessibility of the frames to be consulted, which may already be available on the net or can be inserted by the user himself, who, if at the opening of the program he has not found an answer in the existing material in the catalog, can insert his own frames and choose from:
To detect the coordinates of a photographed point, it is necessary to detect the coordinates of its image on the left and right frames. For this purpose, on the frames it is possible to move (using the keys) a "brand" whose coordinates are known in the reference system of the respective frames.
Obviously, in the case of discontinuous surfaces (which have vertices and edges), it is possible to position the two marks separately on the corresponding points, but in the case of continuous surfaces, such as spheres or cylinders, stereoscopic observation of the frames themselves is indispensable.
In this restitutor the frames are prepared for viewing with:
In practice, the viewer exploits our ability to cross the visual axes: to look at the monitor, through the only opening placed in the center of the screen FF , the observer is forced to cross the visual axes, directing them on the two frames, whose position has been inverted just to make sure that the right eye only sees the right frame and the left eye only the left frame. The central septum G is intended for beginners, led to observe both frames with both eyes.
Terorically, any computer connected to the Internet allows the use of StereoFot. In practice, it should be borne in mind that the program must manage two images with 256 gray levels, with dimensions that vary from a minimum of 800 x 600 pixels to a maximum of 2268 x 1701 pixels, with a resolution of 72 dpi. The software was developed and then tested on a Mac computer with a 1.3 GHz processor, with 4GB 1600 MHz DDR3 memory.
stereocamera and stereoscopic viewer P>
Wild ST4 stereoscopic P> Observing, for example, a pair of frames taken with a base of 3 meters, one would have the vision of a hypothetical giant, whose eyes are three meters apart. Considering that each of us, to better observe an object, takes it in his hands and in this case the relationship between the interpupillary distance and that of the object is on average 1/10, in photogrammetry the relationship between the base and the distance of shooting varies between 1/5 and 1/20, so you can observe the virtual model of the object taken, reduced in scale, at an average distance of one meter. The following diagram shows the model taken with base b1 and the virtual model observed with base b2 , reduced in the same scale as the base. It should be noted that, in the scaling of the model 1-2-3 only the distance between the frames varies, the dimensions of which remain unchanged.
Direct photogrammetry allows you to observe the three-dimensional model of the photographed object, reduced in scale to be observed at a distance of about one meter and allows the relief of the visible points on the pair of frames with:
Taking for granted that the pair of frames has already been chosen from the General Catalog and, therefore, we already have the relevant screen, to carry out the survey it is necessary to:
At the bottom, on the right of the screen are reported: the archive number of the frames, the dimensions of the same on the screen, the reduction coefficient, the shooting base, the main distances, the decentralization, the camera inclination and, for each point detected, the plate-coordinates and the ground-coordinates of the same.
The stereo design
The "draw" button draws, on both frames, a line that joins, in sequence, the points detected and recorded in the scrolling field.
Obviously the graph, observed under the stereoscope, appears three-dimensional.
reliefs of details in the cathedral of Abriola (PZ)
restitution of a couple of frames of the Cathedral of Acerenza
To use reverse photogrammetry you need:
The reference system has:
The stereometric model
This function can be particularly useful for checking the environmental impact of a volume and, with the indispensable use of stereoscopic vision, it takes the following steps:
Two examples for an idea of the virtual stereometric model:
- the virtual arrangement of a pyramidal solid in the test field
- the stereometric modeling of the interior of the Cathedral of Acerenza (PZ)
Measurement of the distance between two points A, B
When you are in "direct" mode and select "representation in the XZ plane" , a panel opens for the representation of the points detected.
Pressing the "Import relief" button, located at the top left, opens the list of existing reliefs in the archive. By selecting one of these, the coordinates appear in the field below the button itself, while the left frame appears, in reduced format, at the bottom, in the right button panel.
With a click on the "show" button , placed under the photograph, this can be called up on the screen. In the absence of the relief, you can use the "import" button (existing on the push-button panel, bottom right), which opens a dialog box, shown below, to directly call up the photograph.
When there are flat surfaces on the frame, perpendicular to the shooting axis, it is possible to scale them. In this case, after detecting the width and height of the surface, the frame can be enlarged or reduced, so as to be brought back to the drawing scale, thanks to two scroll bars, which appear on the right and on the bottom of the plane of drawing. Any graphic can be drawn and digitized on it. To this end, the headstock shows:
The geometric bases
This restitutor has been programmed for the restitution of photogrammetric surveys performed according to the so-called "normal case", ie with the optical axes of the chambers arranged orthogonally to the base (joining the two chambers).
In the planimetric representation of fig. 1 two cameras are schematized and colored in yellow which capture point P, whose image is projected, through the lenses O1 and O2, on the frames (segments colored in blue).
If the cameras are arranged with the optical axes of the parallel lenses and with the coplanar frames, the two formulas shown in the figure can be obtained from the similarity of the triangles, outlined in the figure.
Taking into consideration the Cartesian system XY , we can detect a one-to-one correspondence between the coordinates ( x, y ) of the point P and the linear coordinates x ' and x " (in the frame reference system) of the images of the same point.
Moving on, then, to the spatial representation of fig. 2, from the similarity of the triangles colored in blue, a proportionality between the z dimension of the point and the corresponding representation on the frames is also evident and it is easy to arrive at three o'clock formulas shown in the figure of the so-called "normal case", the conditions of which are: parallel optical axes and coplanar frames .
As far as the c is concerned, it is appropriate to specify the difference between the main distance and the focal length: a metric chamber can have a focal length f = 60 mm. e main distance c = 63.68 mm. The latter corresponds to the distance between the center of projection of the objective and the plane of the frame at the time of shooting. In fig. 3, we take into consideration the similarity between the triangles colored in green, we indicate with A ' is the length of a horizontal rod photographed and with a its image and with C 'the distance of the rod from the camera, with an acceptable approximation, the main distance is given by the formula shown in the figure itself.
The last consideration concerns the units of measurement and possible errors of measurement.
The metric chamber
The metric camera is a camera, equipped with a high quality lens, whose external orientation and internal orientation are known, meaning by:
the Wild C120 stereocamera supplied to the Photogrommetric Unit of the Urban Police of Bari
When you do not have a metric camera and you want to proceed with a common camera, you must take note of the main drawbacks that you face and which can be summarized as follows:
The calibration operations
Four reference marks are imprinted on each metric frame, joining which the Cartesian reference of the frame is displayed, indispensable for detecting the coordinates of the points subject to the return.
In the case of the Wild C120 stereometric chamber, equipped with decentralized optics, the reference marks, present on each plate, are only two, placed at 1/3 or 2/3 of the height, according to whether the decentralization is, respectively, towards the 'up or down (the main point, of course, is located in the middle).
On the cards the decentralization indicated is the opposite of the real one, so that the decentralization " A " corresponds to a higher upward angle and vice versa.
The calibration operations consist in making the frame reference system coincide with that provided by the program.
To this end, by clicking on the " calibration " button, fixed reference marks appear on the frames and a panel with buttons is highlighted ,which allow you to rotate, as well as move horizontally and vertically, each frame, so as to make the brands impressed on the frame coincide with those of the picture.
The data is recorded in special fields, located to the right of the buttons and, in the if the frames are imported, they are stored together with the frames themselves.
With the radio button " DIGITA " selected and the use of the buttons below, the coordinates of each point, where you click, are stored in the second field. This makes it possible to digitize any chart, bearing in mind that to digitize:
The graphic return
If you intend to make a graphic restitution, it is necessary to bear in mind that the photogrammetric survey provides the spatial coordinates (xyz) of all the visible points on both frames.
To represent, for example, the plan of a church, it is necessary not only to make several shots, but it will be necessary to integrate the information necessary to represent the points not visible in the photograph.
To check, therefore, the survey or, simply to take notes, three graphic tablets are available that can graphically connect the points detected, respectively in the planes xy (plan), xz (elevation) and yz (section), more precisely:
The precision test
To get an idea of the achievable precision it is advisable to use the frames of the Heerbrugg test field, supplied with the Wild C120 stereometric chamber, on which there are four reference targets arranged in a "T" and having coordinates in meters:
Once the aiming has been carried out, ie after positioning the mobile brand on a point (practically on both frames), it is possible to register the coordinates with the "detect" button. By clicking on the latter, three scrolling fields appear, where the order number of the point itself, the ground coordinates xyz ( x according to the joining chambers, y according to the optical axis of the left chamber and z orthogonal to both), the plate-coordinates xy (origin at the main point, x axis according to the line joining the fixed marks and orthogonal y axis) and the ground coordinates on the horizontal xy plane.
By restitution we mean the operation with which we obtain the ground coordinates of a photographed point, but the problem of finalization is to be addressed, since the person making the restitution is not the final user of the survey and, therefore, hardly knows what the details are to represent.
The possibility of having the three-dimensional optical model available made the old continuous return unnecessary. In fact, when the restitution took place in the laboratory, using very expensive equipment, observing the three-dimensional optical model, a representation of the photographed object was processed, in orthogonal projection in the xy (plan) or xz (prospect) plane, elaborates that were supplied to the user as the final product. Today, thanks to the digital restitutors, the user himself can detect information from the frames, or report new information on the same frames and we speak, therefore, of:
processing carried out with the Wild A2 universal restitution of a shot made with the Wild P30 phototeodoloite (1974)
Any point, shown on both frames, can be reached in two ways:
The precision of the survey
The degree of precision achievable is, as always, the result of a compromise.
To be taken in first consideration, there is the pointing error: even with the aid of stereo vision, everyone places the mobile brand on the point to be detected in a completely personal way.
Taking for granted that, during the recovery phase, the operator has never forgotten, before shooting, to fix the plate on the support panel, to this type of error we must add a series that we could define systematic and that, in the particular case , we could summarize as follows:
Origin of the xyz reference system
During the recovery phase, the origin of the xyz system of the ground coordinates is fixed at the recovery point on the left, therefore, even during the return phase, by default, the position of the origin remains unchanged and in the window, which appears after clicking on the "detect" button, we find, in the first field, the values (x, y, z), that is, the ground coordinates of the detected point.
In "reverse" mode, you can move the origin of the reference, where the mobile brand is located, using the " new origin " button. In this case, a dialog box asks for confirmation and, if so, the survey already carried out is deleted from the field, to make room for the survey with the new origin, the coordinates of which with respect to the original reference, are shown in the appropriate fields.
The mobile brand
The mobile brand, intended to be placed on the point to be detected, is represented by a small circle with four dashes, arranged in a cross and yellow, when using direct photogrammetry, and by a yellow-black pole, when reverse photogrammetry is chosen. As for the movements of the same, it should be borne in mind that:
The metric frames
In photogrammetry the terms "photogram" (and not "negative" or "diapositva" or "photograph") and "plate" (and not "film") are used, to remember how indispensable the "metric" characteristics are.
The scheme above. points out that the ground coordinates of a point P depend on:
With the "import" button it is possible to import, from the server archive, the ground coordinates and the plate coordinates (which will be arranged in the respective fields) of the previously detected points.
When clicked, a field appears containing the list of findings in the archive, among which it is possible to choose the document to be imported.
With the "export" button it is possible to export, in the server archive, the ground coordinates and the plate coordinates (contained in the respective fields) of the detected points, after assigning a name to the document in the dialog.
The document can be imported later, to continue or complete the survey operation.
The program displays, with a click on "detects" , the plate-coordinates of the mobile brand on the two frames. The fields indicate:
Filmed with the Wild C120 stereocamera