- The planning of Flight Mission
The planning of flight mission is based on the center of a 1/5000 aerial orthophoto base map. From the west to the east, line out 80 flight lines over the whole Taiwan main island. Average ground resolution: 25—30 cm.
- The Frequency of Aerial Photographing
On plain area: We carry out two times of aerial photographing on plain area every year.
Mountain areas: Since the ground surface seldom changes on the mountainous areas, aerial photographing is carried out every two year.
- Activities in the sky
365 days around the year, aerial photographing will be carried out as long as the weather conditions are good.
On special demands: for example, standby anytime for conducting terrain investigation after a natural disaster.
Regular time for aerial photographing: considering the terrain conditions and shadows caused by the angle of the sunlight, the best time for conducting aerial photographing is 10am—14pm. Adjustments will be made according to the weather and flight line conditions on duty day.
- The Target of the Aerial Survey
On an aerial photo image, the controlling point must be a sharp image point which can be easily identified. The best way to reach this goal is to set up Aerial Photogrammetric Targets on the ground before conducting aerial photographing activities. When setting up aerial targets, it is necessary to choose positions which are clear and wide, without any shadows, and properly distributed within the surveyed area. In this way, it would be better to receive GPS signals and take aerial photos that meet the requirements of map production.
- Control Survey
After intensive surveys and calculations, related data can serve as the basis for map production. Along with the advancement of the survey technologies, traditional surveys by way of triangulation and trilateration have been replaced by the present surveys with GPS, which can obtain highly precise results in a short time.
Photos record the information of objects, landform in a 2D image. To observe and measure two adjacent photos with correct orientations in an stereo viewing equipment, the 3D-space can be reconstructed.
■ Aerial Photo Triangulation
The coordinates of the control points on the ground provide the absolute positions for aerial triangulation. Based on this information, other unknown coordinates of the conjugate points in other models can be calculated. The exposure (center) position and attitude parameters of the aerial photos can also be obtained.
■ Topographic Mapping
Stereoplotting is carried out based on the adjustment result of aerial triangulation and aerial photos together with a stereoplotter which can show 3D models. By measuring and digitizing the contour lines, topographic structure lines , and structure lines of objects, the landforms and ground objects observed in the models are recorded and saved as digital files so that they can be applied to other tasks.
- Rectifying Geometric Properties of Aerial Images
Photos can show 2D images only, with the phenomena of relief displacement and tilt displacement due to the different height of landform, landscape, and objects. Besides, the situations of aerial photographing also vary a lot. For these reasons, the map measuring can’t be taken directly in photos until the geometry was corrected. The orientations of the photos must be solved by aerial triangulation, and correct the image geometry by procedure called ortho-rectification.
A procedure to modify or rectify an aerial image from perspective projection (or central projection) to parallel projection. According to the orientation parameters of each photo and corresponding DTMs, a pixel-wise differential rectification will be proceed to each aerial image. The purpose is to eliminate the relief displacement or tilt displacement in the images. In this way, digital image files of ortho-images with coordinate system and identical scale are created.
Ortho-images can be applied to related fields of the GIS system, such as land planning, resource managing, agricultural and forest resource surveying, construction of railroad and highways, river regulation and management, environment monitoring, as well as fire control and rescue, etc.
- Aerial photographs which are reconstructed into 3D must be marked with necessary information before they become usable graphic data.
The duty of graphics compiling mainly focuses on the post-production work of ortho-images, decorating the whole image before printing, and quality controlling.
■ Post-Production Work of Ortho-Images
The tasks include mosaicking of ortho-images, adjusting the tones or hues of images, and trimming the images into the size of 1:5000-scale map sheet.
■ Decorating the Whole Image and Quality Control
The major task is to transfer the ortho-rectified aerial images into graphic languages which can deliver messages so that these images become easy-to-read graphic data. In other words, it is to express geographical objects by using map symbols, such as adding symbols, icons or notes. In addition, data outside the frame must also be added, including sketching contours of administrative sectors, declination diagram, location maps, and the relative positions of the connecting parts. After that, quality control activities such as verifying the map will be carried out repeatedly in order that the users can quickly obtain the correct information they need when using the images.
5.Providing Graphic Data
- Since the 1970s, the Aerial Survey Office has been carrying out aerial photographing activities ceaselessly and has taken detailed photos of each inch of Taiwan’s land. Thus, everything happened on this land – whether they are the traces of prosperity, the scars caused by disasters, or each change caused by time – all can be found in the constantly collected aerial photograph of our office.
■ Conserving Graphic Data
◆The Negative Films of Aerial Photograph
Our office has conserved more than 810,100 pieces of photos in 3,747 rolls of negative films during the period from 1973 to 2007.
◆ 1/5000 Orthophoto Base Maps of Forest Areas
◆ Orthophoto Base Maps of Forest Areas
◆ Black-and-White Orthophoto Maps
Totally more than 6,000 pieces
◆ Colored Orthophoto Maps
Totally more than 50,000 pieces since 2001
■ Digitalizing Traditional Aerial Photograph Data
In order to efficiently conserve and manage the traditional aerial photograph as well as reduce the damage of original negative films, we have introduced 4 precise digital scanners – INTERGRAPH Photoscan TD – since 2000 with a resolution of 14μm to scan and file all the negative films of original photographs taken before 2007. Up to July 2010, we have completed the scanning of 811,732 pieces of photograph in 3,747 rolls of negative films.
■ Data Saving
To meet the requirement of the digitalization of aerial photographing, we also took necessary measures to store and protect digital data. In addition to purchasing large-volume saving devices, distant backup systems were also established to make sure that all the valuable graphic data are saved safely.
◆ Data Files Saving System
There are two kinds of large-volume saving devices in Taipei.
Hard disk: 250TB、300TB and 320 TB each.
Data backup device: one 130TB hard disk and one 1,080TB tape.
◆ Distant Backup System
The graphic data and digital data of our office are also saved in the National Center for High-Performance Computing.
■ Interpreting Aerial Photographs
Interpreting aerial photographs is to differentiate the ground object and to identify its shape, size, height, and other attributes by aerial photographs. Because aerial photographs are taken in high flying height, the images on the photographs are very different from what we see on the ground. Thus, interpreting these photographs needs not only 3D glasses but also certain professional knowledge as well as abundant experiences.
■ Agricultural and forest resources aerial survey
Agricultural and forest inventory have been carried out through photogrammetry method and after land use type maps produced based on orthophoto base map, computerized aerial survey data system are employed for data statistics and land use type map reproduction to establish land use fundamental data bank.
■ Remote Sensing
Remote sensing is a highly efficient survey technique. Color infrared aerial photos and multispectral scanning data are employed by this Institute to help implementing land use survey , identify tree species , detect agricultural and forest damage caused by natural disaster , monitor disease and pest damage of rice and forest . Besides , color institute to other government agencies and school concerned for remote sensing research and development.
7.ASO Taiwan Image Services, Supplier, and System (ATIS3)
Due to its specialties, the Aerial Survey Office has the richest and most complete images database of aerial photographs in Taiwan. The abundant images of Aerial Survey Office are valuable resources and represent strong demands.
However, due to the limited human resources and the large amount of photographs, it is very difficult to provide aerial photographs efficiently, limiting the provided service. In addition, the demands from governments, academics, and research institutions have been constantly increasing. In coordination with the development of the Internet and GIS technologies, related institution such as the Open Geospatial Consortium (OGC) has defined the Web Map Service Interface Standard (WMS).
With the technologies, the system of providing images through the Internet was gradually established. It also provides a new way for the Aerial Survey Office to deploy and broadcast new images. Although aerial photographs can be deployed and broadcasted via the Internet, the enormous number and large volume of images makes it challenging to deploy all these images with WMS. Deploying all the images manually will be time consuming and fallible. After ATIS3 had finished establishing, deploying and broadcasting images are performed with batches, and human resources could be reduced as much as possible, saving time and reducing errors. The quality and capacity of service are significantly enhanced.