Photogrammetric Mapping

A case study of El Salvador City has been done using various photogrammetric techniques. This project comprised the complete new mapping of the town including all features like buildings, boulevards, roads,manhole/ drainage, light / telephone polls, trees /tree areas, ponds, river and streams (with break lines), Monuments, playground / Parks etc. The vectorization of the model has been done on Digital (Socket Set) and analytical systems both. From speed point of views digital systems are as good as analytical systems. It also depends on the skills / expertise of the Operator who is preparing the maps.

Material used Two Diapositive with a scale of 1:5000, Two scanned images with 14 micron per pixel and two images with a resolution of 24 micron per pixel. Scale of Photographs: 1: 5000

Methodology

An appropriate method for assessing the accuracy of derived data and the impact of parameters on the data is to carry out tests using independent check data in the object space. Quality control, or checkpoints with a known elevation considered to be true, were used for accuracy assessments and were measured using DGPS (differential global positioning system). Diapositives are used in Analogue (A8 & AMH) and Analytical plotters (BC3, SD2000).

To start a project on analogue/ analytical systems, Ground Control Points (CP File) is required or created for the entire area / model. In these plotters one MD ?le (orientation parameter ?le) is created for every model using control points.

In Analogue Machines, orientation is totally manual. Inner Orientation is done on a light table where four ?ducial points are brought in ?ducial axis marked on the picture carrier plate on which the diapositive is mounted. Later, it is mounted on instrument for RO and AO. In Relative
Orientation Y-parallax are removed using Kappa, Phi and Omega. After that Absolute orientation is performed where Known GCP’s are measured. It takes more than half an hour to orient a single model.

In Analytical Plotters, orientation is automatic. For making orientation parameter ?les (MD Files), ?rst of all, diapositives were mounted on instrument and started with Inner Orientation. It starts with left image followed by right image. These are automatic instrument even then coordinates at ?rst two ?ducial points were measured manually. Later it reads remaining six ?ducial points automatically. The same process is done for right photograph. The RMS value of IO should be less than 10. Epipolar Orientation follows inner Orientation where parallax is removed manually at two points. After the Epipolar orientation, Relative Orientation is performed. During RO process, Y parallax is removed at least at six points or maximum at 10 points. For Absolute Orientation, ground control points are measured and RMS is noted. After AO process, model becomes ready for data extraction / map production.

For Digital Photogrammetry, photographs were scanned, using an LH System’s photogrammetric scanner, at a resolution of 14 microns and 24 microns per pixel prior to reformatting and transfer into the Socket set Digital Photogrammetric Workstation (DPW). Full details of camera calibration parameters were supplied to enable interior orientation to be carried out. All processes like IO, RO and AO are done for digital systems also. Stereo model were constructed using premarked ground control points positioned using post-processed differential GPS. Subsequent analysis involved the calculation of residual values produced by comparing the computer-generated surface with a set of test points measured using differential GPS. This research demonstrates that the optimal digital system matched the performance of analytical photogrammetry for the feature extraction.

Observations

After AO process, RMS is checked. About 20 points were observed in Analytical and Digital system. Later these points were compared with known GCP’s.

Following RMS values are obtained in Analytical system:

Planimetry = 14.0 cm

Altimetry = 10.0 cm

Following RMS values are obtained in Digital system:

Planimetry = 17.0 cm

Altimetry = 12.8 cm

The above result is from photographs with a scale of 1:5000

Note: – If photographic scale is small like 1:25000, then RMS may be different. Planimetry accuracy may go around 50 cm and Altimetry accuracy may go around 80 cms.

Discussions

For stereo compilation, the Analytical plotter is a clear winner. For largescale mapping, softcopy systems couldn’t resolve the ?ner details that Analytical plotters could do.Through observation it has been found that Analytical systems give better accuracy up to 5-10 cms whereas digital systems give an accuracy of 09-13 cms. Since diapositive has continuous tone so it doesn’t get blurred while zooming and gives better view and results whereas digital images can’t give the same results as these are having pixel. The size of the pixel of these images depends on the scanning (DPI) and resolution of Photographs. If one image has a pixel size of 14 microns (Scale 1:10000) and other image has pixel size of 24 microns (Scale 1:10000) than we would get better accuracy with the image having pixel of 14-micron size. That means if the resolution were better, the accuracy would be better.

With regards to Image Quality the vote went in favour of the Analytical plotters although several users said that the difference, particularly on the higher resolution (15 microns) was not that great.

For DEM generation, there is a positive feeling amongst users that softcopy system are as accurate or better than Analytical plotter and that correlation gave the softcopy system a distinct advantage.

Aerial triangulation is also an area where the softcopy system performed well and many users stated that they achieved faster and easier production. If the only function of softcopy was stereo plotting, it appears that their impact would be considerably less than we are experiencing. However, the softcopy systems are more in use and covers in many areas of map production in a single system.

Through softcopy systems, one has the option for number of applications, all in the same environment (compilation, DEM, draping of contours, orthogenertion etc.) and softcopy systems are less expensive.

Results

Evaluated the accuracy of orientation parameters as well as the quality of datasets of analytical and digital systems. Also checked the data accuracy obtained through images with a pixel size of 14 microns and 24 microns. Dataset from 24 microns per pixel is of inferior quality.

Production Accuracy

From accuracy point of views, for stereo plotiing, Analytical Plotters give better accuracy as compared to Digital Workstations. Regarding DTM generation, softcopy can be considered better.

Conclusion

In Photogrammetry, aerial data is being used for Mapping and Up gradation of urban sprawl. Both digital and analytical techniques offer potential for those interested in recording surface features. However, selection of the technique needs to be based on a careful consideration of the end products required. In general, this research shows that digital photogrammetry performs well but comparative analysis of Analytical and digital dataset, on the basis of equal resolution, shows that analytical instrument give a better planimetric and altimetric (Z-value) accuracy than Digital workstations.

At present, both analytical and digital systems offer solutions for utility mapping and research. However, as shown in this case study, the digital data capture systems can’t compete on accuracy terms with analytical systems but offer many potential advantages in terms of data collection time and other end products, which are available. However, despite the increasing user friendliness of many of the digital systems, a high level of expertise is still required.

References:

1 Text Book of Photogrammetry, P.R Wolf

2 Manual of Photogrammetry, American Society of Photogrammetry, Fourth Edition

3 State of the Art of Digital Photogrammetric Workstations for Topographic Applications, C.Helpke,Photogrammetric Engineering & Remote Sensing ,Vol.61,No.1,Jan1995,pp49-56

4 IIRS-1C/Dstereo Data Processing System-Operartion Manual for DEM and Orthoimage Product Generation Software (Dat Version) for IRS1C/D, Signal and Image Processing Group, Signal, Image and Information Processing area, SAC, Ahemdabad.

5 Concepts and Algorithms in Digital Photogrammetry,by T.Schenk,OEEPE Journal,Vol.49, No.6,Dec.1994

6 Earth Surf, Process. Landforms 28, 307–320 (2003)

7 Proceedings : Mapping and Remote Sensing Tools for the 21st Century, conference and exposition with feature emphasis on Public / Private Interaction, august 26-29, 1994, Washington D.C.

Advantages of softcopy photogrammetry

This comparison of analytical and digital techniques has shown that Digital Photogrammetry have greater advantages to analogue / analytical plotters as better stability (geometry and radiometry), good accuracy, and the ability to process images. It is also possible to display vector data in the images on the screen and to produce transformed images (recti?cation, Orthophoto). Today the handling and use of digital images on standard PC’s is possible up to the ?nal production of orthophoto mosaics and contour lines without problems. Digital Photogrammetry Workstation offer other additional products including ?y-through, image drapes, view shed analysis tools, orthophotographs and image analysis tools.

There are several other advantages connected with using digital images which are as follows:

• Production Ef?ciency is very good.
• No Instrument calibration is needed.
• Measurements are automatic and faster.
• No repetitive IO, RO & AO to start a model.
• Stereo images can be reconstructed very fast.
• DTM and Orthophoto generation is automatic.
• Single model as well as strip / block orientation is possible.
• No constraint of focal length bracket as in analogue system.
• Digital images are more stable as compared to ?lm material.
• Integration with Remote Sensing facility, real time data updation.
• 3- D graphical superimposition facility for automatic feature extraction.
• Automatic measurements possible with correlation techniques.
• Digital image processing techniques can be used for image enhancement.
• Simple to Operator, no wear and tear of mechanical and optical parts compared to Analogue and Analytical Instruments.