DIGITAL PHOTOGRAMMETRY MADE AFFORDABLE

Transcription

1 DIGITAL PHOTOGRAMMETRY MADE AFFORDABLE Allen, S. SmartTech. 34 Firgrove Way, Constantia Hills, Cape Town Fax: Website: ABSTRACT The photogrammetric mapping industry is currently experiencing a noticeable increase in the number of Digital Photogrammetric Systems (Softcopy Systems) being introduced to the market. These systems are often marketed at between 10 and 20% of the price of analytical machines and/or Softcopy Systems that have been around for 10 years or longer. Quite justifiably the following questions could be asked:! Do these new inexpensive systems have the same functionality as the older systems?! Is it now more cost effective to produce plans and maps than before and if so could I use photogrammetry to speed up and reduce costs where I previously used conventional ground survey techniques? The major components of a Softcopy System are computer hardware and software. Most often Softcopy Systems require hardware that is over and above the standard desk top PC such as a suitable stereo enabled graphics card, an accurate user control device e.g. hand-wheels, foot-disk etc. The software (or computer program) is often written to interface directly with hardware and firmware for efficient functionality e.g. the graphics card or joystick. There is no doubt that more and more individuals and companies are entering the Photogrammetric market quite probably due to the recent affordability of Softcopy Systems. Yet even well established photogrammetric companies are struggling to come to terms with the rapid changes in the hardware and software related to Digital Photogrammetry. This paper therefore takes a critical look at both the hardware and software requirements of an efficient affordable Softcopy System. When analysing the hardware component we will look at items such as stereo vision, panning speeds, refresh rates, control devices and user comfort. Integral components of this are the graphics card, monitor(s), processor speed, memory (RAM) and hard drive. Short and long-term storage requirements of very large images will also be covered. In our analysis of the software component we will look at user friendliness and functionality. Integral components of this are items such as the operating system, user configuration options, project set up, camera and lens information, inner and exterior orientation, bundle block adjustment procedures, the mapping application, automatic correlation and orthophoto creation. In short, the reader/attendee interested in photogrammetry will be able to make an informed decision on the requirements for a cost effective and accurate Softcopy System thus enabling him/her to do both aerial and close-range mapping at an affordable price. 1. THE HARDWARE REQUIREMENTS OF AN EFFICIENT SOFTCOPY SYSTEM Unlike analogue or analytical stereoplotters, Digital Photogrammetric Workstations (DPW or Softcopy Systems) require minimal hardware and take up far less ground area (footprint). For many years the computer systems utilised for Softcopy Systems were considerably above average in power and were therefore relatively expensive. It was not unusual for example for these systems to have dual processors with Random Access Memory (RAM) way above the average Personal Computer (PC) at the time. Even with the amount of power available these systems were often slow and gave intermittent pauses (reloads of image data etc). Many experienced operators considered them inadequate for extensive mapping operations. Proceedings of the 21 st International Cartographic Conference (ICC) Durban, South Africa, August 2003 Cartographic Renaissance Hosted by The International Cartographic Association (ICA) ISBN: Produced by: Document Transformation Technologies

2 Figure 1. Operator, stereo images and polarised screen. However this situation has changed dramatically over the past few years with processing power now doubling at a phenomenal rate (at least within each year). This has made very powerful machines highly affordable which in turn has improved the attractiveness of Softcopy Systems in general. Many Softcopy Systems therefore now only require a Personal Computer (PC) to run on. Nowadays the PC used therefore often looks very similar to any other home/office computer although typically a top end system is obtained. Hardware specifications are often outdated by the time they get to print but with efficient software, absolutely smooth panning and subsequent efficient processing will be achieved using the following guidelines: Figure 2. Motherboard, processor and memory Processor: 2.4 GHz Motherboard: On board sound, LAN, USB 2 and RAID. Memory: 512MB or 1GB for large colour images Mass Storage: 2 * 120MB hard disk drives mirrored by RAID for security and speed.

3 Graphics card: Dual head graphics card with 3 pin mini DIN for stereo glasses/polarised screen. VDU: 2 * Monitors capable of running at a resolution of at least 1024 * 768 at a refresh rate of 120Hz. Other: CD or DVD reader/writer, floppy disk, keyboard, mouse, joystick etc. All of the above hardware can be obtained currently (April 2003) for around $ For stereo viewing there are basically 3 options available:! Mirror stereoscope ($550.00).! Active 3D glasses ($299.00).! Polarised screen ($ ). Some users also prefer to use the optional hand-wheels and foot-disk which gives the system the feel of an analogue or analytical machine. These cost around $ CHOOSING THE BEST HARDWARE FOR USER COMFORT As previously stated there has been a recent upsurge in the usage of Softcopy Systems. It is now quite common to do full projects from start to finish on a Softcopy System. To achieve this objective it is of paramount importance that operators feel at least as comfortable and even more comfortable when using a Softcopy System as compared to an analogue or analytical system. The 2 primary components of this user comfort with regards to the hardware is the smooth panning of the images and combined with this, the stereo visualisation method. 2.1 Smooth image panning There are 3 major components influencing the panning speed. They are:! Graphics card: this determines how fast the data is received by the card and displayed. A slow graphics card can create a bottleneck thus causing jerky panning.! Memory (RAM): an efficient Softcopy System will buffer as much image information (raster data) as possible to avoid excessive disk reads. Limited RAM will cause System Page Faults which the user will see as pauses in the panning. On some Software Systems this can be for considerable periods e.g. up to 30 seconds although on efficient systems there should only be a very slight pause e.g. a quarter of a second or less. To avoid this the system should have approximately 200MB + 2 * 60% of the average image size.! Processor: the speed of the processor determines how fast the new data is computed, gathered and sent to the graphics card. Here the biggest effect is seen if vector data is displayed in stereo with the raster data i.e. superimposition. A slower processor will cause the new data to be computed too slowly for fast and smooth panning. 2.2 Stereo Visualisation Methods As previously stated there are basically 3 viewing methods available. Their advantages/disadvantages will be covered individually in detail: Figure 3. Stereoscope attached to monitor

4 2.2.1 Mirror stereoscope With this method a mirror stereoscope is mounted on or in conjunction with a computer monitor (VDU). Two individual images are displayed on a monitor side by side and the mirror stereoscope is adjusted so that each image enters the left and right eyes separately. In this way a stereo image is seen in a similar manner to an analogue or analytical stereoplotter. Advantages Cost: generally this is a very cost-effective system as low cost graphics cards and monitors can be used Robustness: There are no electronic parts and the stereoscopes are simple to operate Disadvantages The operator has to sit close to the eyepiece in a similar manner to an analogue or analytical stereoplotter Only half the amount of image data can be displayed as when compared to the other methods as the 2 images are placed side by side Figure 4. Operator with stereo glasses Active stereo glasses With this method liquid crystal glasses are worn which are electronically shuttered at a frequency of around 120Hz. The right eye is therefore effectively closed while the left eye observes the left image on the monitor. The reverse is then done with the left eye being closed while the right eye observes the right image. This is done very rapidly so that the user does not perceive any visible flickering in front of the eye. Advantages Better stereo perception because of the larger viewing area The operator can relax and see stereo from virtually any position Considerably less expensive than the polarised option Disadvantages Operators can experience visual fatigue and sometimes even headaches with prolonged use The glasses are heavier than the polarised option and can become uncomfortable The glasses are easily damaged Infrared emitters can interfere with each other if many systems are used in the same environment

5 Figure 5. Operator using polarised filter Polarised filter (Z screen) With this method a polarising filter is attached in front of a monitor. The operator wears normal polarised sun glasses and the left and right eyes are effectively blocked by polarising the filter in a direction opposite to the relevant eye. This effect can be seen by placing 2 polarised lenses at right angles to each other whereby no light passes through the 2 combined lenses. Advantages Very light sun glasses which operators often forget that they are even wearing Normal tasks can continue without any interference e.g. writing, etc. There is no interference from neighbouring systems Clip-on Polaroid filters can easily be added to normal prescription glasses Low maintenance costs and inexpensive to replace glasses if they are damaged Cost Disadvantages 3. ADDITIONAL HARDWARE As with analogue/analytical systems, some form of control is required to hold the floating mark on the ground when manually mapping. This device needs to have 3 dimensional control as well as various buttons which should be user configurable for various commands. If a hand-held device is used then it should be effective yet be affordably priced. If hand-wheels and foot-disks are chosen then they should also be affordable i.e. in keeping with the cost of the system. It will be shown later that digital images have considerable disk storage requirements with a typical project ranging from 10 to 100 Gigabytes or even more. Long-term storage on magnetic media (e.g. hard disk drives) is not ideal as

6 data could be lost over extended periods. A good addition to the hardware of an efficient Softcopy System would therefore be a DVD reader/writer. The disks take up little space and will store data safely for extended periods. Figure 6. Wingman, hand-wheels, foot-disk and foot switches 4. THE BASIC SOFTWARE REQUIREMENTS OF AN EFFICIENT SOFTCOPY SYSTEM As previously stated there are many more Softcopy Systems now available on the market than there were for example 10 years ago. Potential users are often left with a confusing choice. Without careful scrutiny of the various systems available and their functionality, wrong choices can easily be made. This often leads to the unexpected requirement for further modules with the final cost of the system escalating on each additional purchase. Here follows a guideline for the basic requirements: 4.1 Overall user friendliness Here are some examples to look at with emphasis on the graphical user interface (GUI) and navigation around the various routines:! Ease of project set-up and swapping between projects.! Ease of lens and camera set-up.! Automatic processing and conversion of images.! Ease of inner orientation set-up, with manual and automatic fiducial mark measurements.! Ease of aerial triangulation measurement and bundle block adjustment with interaction between the computation and measurement routines.! Ease of exterior orientation set-up, with manual and automatic point measurements.! Ease of feature capture and the mapping/gis tools available. 4.2 Stereo visualisation Caters for the following options, which the user can change at any stage:! Split window/screen; not only for the mirror stereoscope option but also for ease in exterior orientation etc.! Anaglyph stereo for training purposes etc.! Active stereo! Passive stereo

7 Figure 7. Anaglyph stereo images 4.3 Image panning speed (smoothness) More on this later. 4.4 Cameras Have the ability to cater for non-metric or inexpensive digital cameras. 4.5 Close range/non-vertical photogrammetry Allows the user to process non-conventional aerial images i.e. where the camera is held at any user required angle e.g. for bridge site measurements, volumetric calculations etc. The user should then be allowed to use any Cartesian coordinate system without the need to swap coordinate axis etc. All model data (images) must be processed/combined in a single coordinate system. Final output data should also be configurable to any required coordinate system. Figure 8. Close range Photogrammetry, stereo pair 4.6 Aerial Triangulation observation and computation Allows at least 6 images to be displayed simultaneously for tie point transfer between models and strips. Bundle block computations should be included in the software to allow iterative computation and re-observation instantaneously within the system i.e. without the need to export to external ASCII files etc.

8 4.7 Interior orientation Ability to import and export from and to other systems. 4.8 Exterior orientation Ability to import and export from and to other systems. 4.9 Superimposition Allows the vector data to be displayed in conjunction with the raster data while busy mapping and in stereo. This is not the same as having the vector data combined with the final ortho-rectified image i.e. the user must be able to see the vector data combined with the images in stereo so that it is easy to distinguish which detail has been mapped etc Mapping Allows the user to map directly into the final CAD/GIS system. This must be seamless i.e. across models etc. The system should change and activate models automatically as the user moves across the terrain Autocorrelation (automatic measurements for at least the following)! Inner orientation! Relative, model and strip connections.! Grid or DEM measurements. At a minimum 50 points per second (preferably faster) with user parameters to control spikes, exclusion areas, iterations etc Triangulated Irregular Network (TIN) or surface creation Supply routines for the generation of break lines, TIN s, automatic contouring etc. for an unlimited number of points Orthophoto creation (the system must at a minimum cater for the following)! Orthophoto creation in an unattended batch process across as many models as the user requires and offering all of the following user selectable techniques:! Nearest neighbour! Bilinear! Cubic Convolution! Automatic balancing and feathering of the individual othophotos to form a totally seamless image without any visible seam lines or tonal differences.! Allow the user to specify shapes of any size and form for the final user defined orthophoto(s). 5. IMAGE HANDLING AND PANNING SPEEDS It is clear that smooth image panning is of paramount importance in a Softcopy System. If a system is not at least as effective as conventional analytical machines then the Softcopy System will have failed in its most basic task and operators will be reluctant to change. One needs to look at the approximate sizes of typical metric digital images to see the need for efficient software:! Grey scale image scanned at 25 microns = 80 Megabytes! Grey scale image scanned at 12.5 microns = 320 Megabytes! Colour (RGB) image scanned at 25 microns = 240 Megabytes! Colour (RGB) image scanned at 12.5 microns = 960 Megabytes (nearly 1Gigabyte) From this it can be seen that the software needs to be able to handle very large images efficiently. It should also be remembered that 2 images are required to make a stereo pair. Much time could be spent attempting to justify various techniques for achieving efficient display/panning speeds such as:! Tiling! Overviews! Compression techniques! Background buffers! etc However after all is said and done the final proof is in simply testing the Softcopy System to ensure that it pans at a smooth and comfortable speed. The user should therefore look out for the following:! On opening, the Softcopy System should virtually instantaneously display the stereo images at the last position observed before the system was shut down regardless of the size e.g. 1 Gigabyte for each image.

9 ! Image buffering (caching) to memory should be done in a background process i.e. with as little interference to continuing work as possible. From a practical point of view, this means that it should be able to pan the images immediately after the initial area is displayed.! Zooming in and out must be virtually instantaneous regardless of the size of the image.! The 2 images must pan as smoothly as an analytical machine yet with all the advantages of very fast panning, zoom in/out etc. across the model.! It must be possible to zoom in further than the ratio of 1 image pixel being equal to 1 screen pixel (1 to 1). 6. SOFTCOPY ADVANTAGES OVER ANALOGUE/ANALYTICAL There are many advantages in using Softcopy Systems when compared to analogue/analytical systems such as affordability, compactness, portability, stereo superimposition, ease of use etc. However there are 3 distinct advantages that deserve special mention: Figure 9. Stereo pair with raster and vector data 6.1 Mapping speed Here advantages can be seen right from the inner orientation to the final vector mapping.! Inner orientation is mostly done automatically and only once for each image i.e. never repeated as is the requirement with analogue/analytical systems.! Relative/Absolute (exterior orientation) is mostly done automatically and all rotations (e.g. kappa) etc are automatically computed and applied even if the user opts to read manually. Driving to points is instantaneous.! Stereo superimposition is a major advantage as the operator does not have to look away from the stereo image to confirm areas that have or have not been mapped. Probably the greatest speed advantage is the ability to map across models totally seamlessly. The usmart system for example switches models automatically so the whole job is effectively viewed as one large stereo image, e.g. operators can capture a road edge across as many models as the road runs without any intervention or boundaries imposed by individual models i.e. truly seamless mapping. Figure 10. Surface created using automatic DEM

10 6.2 Auto-correlation This principal can be used for various tasks but one for which it is widely used is automatic DEM creation. Here the operator specifies an area, row and column spacing, and the software will automatically create a DEM. Some advantages are:! Up to hundreds of points per second.! Greatly reduces the mapping tasks and is highly effective for orthophoto creation etc.! The operator can control height tolerances, exclusion areas, distances from manually placed points etc.! The area can be set to cover multiple models and therefore run in an unattended batch mode. Optimum hardware usage is therefore achieved by running the system after hours. Figure 11. Orthophoto 6.3 Orthophoto generation Here it takes very little operator set-up to create a truly seamless and perfectly rectified image. Some advantages are listed here:! A more intuitive map is created which can be much more easily understood by non-map literate users! Each pixel is geometrically rectified so the orthophoto is as accurate as the vector data and DEM.! After set-up, a totally automatic procedure produces balanced and feathered images that can be cut/merged into any shape or size. 7. CONCLUSION From this it can be seen that Softcopy Systems have become highly attractive for accomplishing tasks which previously would have been done by conventional ground survey techniques. There are many possibilities but here are just a few to consider:! Mapping stockpiles etc for volumes.! Mapping smaller projects using, for example, balloons to suspend the camera.! Precise mapping of, for example, engineering features e.g. cars, engineering parts etc.

11 DIGITAL PHOTOGRAMMETRY MADE AFFORDABLE Allen, S. SmartTech. 34 Firgrove Way, Constantia Hills, Cape Town Fax: Website: Biography Spencer Allen is married with 2 children and his extramural activities include league squash, cycling, music, woodwork and building restoration. Educational Qualifications Spencer was educated at Plumstead High School from 1974 to 1978 before going to Technikon where he completed the National Diploma (Civil Survey) , the National Higher Diploma and the B. Tech. Degree in 1996 (graduated with distinctions). Employment History Spencer was employed by the Provincial Administration (Roads) from 1979 as a student to 1996 where he held the position of Chief Industrial Technician doing research and development. This position involved the computerisation of the section called the Survey Subdirectorate of the Provincial Roads Department. This involved researching hardware requirements and developing software for the various applications. C was utilised extensively on the PC with the DOS operating system. He developed a package which did traverse reductions, least squares co-ordination of points, file manipulation, horizontal alignment calculations of roads etc. He used OPL (operators programming language) on the PSION data logger to capture and process data from theodolites in the field via RS-232 ports. He developed a complete package which performs all the necessary calculations and logging the surveyor needs to do in the field, as well as transferring the information to PC for further processing. He then developed a CAD mapping and road design suite of programs called SMART which co-exists with MicroStation. This was written in MDL (MicroStation Development Language) which is a dialect of C. In 1996 Spencer started his own company called SmartTech. usmart Mapping Software (developed by Spencer) has since expanded and the latest module is a full Digital Photogrammetric System (usmart Softcopy System) which is widely used overseas and locally. Currently Spencer is an active director of SmartTech, continually developing new features for usmart, ensuring it remains the most competitive product of its' kind in the marketplace.