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THEOS (Thailand Earth Observation Satellite), the first Earth observation satellite of Thailand, was successfully launched by Dnepr launcher from Yasny, Russian Federation, on Wednesday, October 1, 2008 at 06:37:16 UTC (Universal Time Coordinated) or 13:37:16 hours, Bangkok Time.

THEOS, Thailand’s state-of-the-art satellite imaging system, is operated by the Geo-Informatics and Space Technology Development Agency (GISTDA) (http://new.gistda.or.th/en/), Ministry of Science and Technology. The satellite’s sensor and orbit characteristics have been chosen strategically to provide worldwide geo-referenced image products for interdisciplinary applications including agriculture, forestry, land use, environmental monitoring, cartography, water resources, coastal zone and natural disaster management. With its oblique viewing capabilities, THEOS can support timely image of any part of the world in less than 3 days, with the resolution of 2 meter in panchromatic and 15 meter in multispectral images. The THEOS visible blue band is similar to that of the Landsat Thematic Mapper while the visible green and red and the near-infrared bands are similar to those of the SPOT family of imagers.

The following tutorial outlines the steps required to orthorectify THEOS System Corrected Level 1A data in Geomatica OrthoEngine V10.3

Rigorous Modeling
OrthoEngine is capable of performing Rigorous Modeling using THEOS System Corrected Level 1A data. Model calculations can be achieved without collecting Ground Control Points (GCPs) thereby orthorectification can be achieved on the fly.

1. Initial Project Setup
Start a new project and select the math modeling method as ‘Optical Satellite Modeling’. Under ‘Options’ select ‘Toutin’s Model’. After accepting this panel you will be prompted to enter the projection information for the output files, the output pixel spacing, and the projection information of GCPs. Enter the appropriate projection information for your project.


2. Data Input
You do not need to import THEOS Level 1A imagery into a PIX file using ‘Read from CD-ROM’ as this type of data is supported directly in the Generic Database Library (GDB). The image can be added to the project by selecting the DIM file in the Data Input stage. Simply click on the ‘Read from PCIDSK file’ icon, then select the file from the correct directory.

Note: You will need choose to see all File types (‘All Files (*.*)’) in order to be able to see the DIM file format.


3. Collect GCPs and Tie Points
While GCP collection is optional, it is recommended for the most accurate results. Select the ‘GCP/TP Collection’ processing step. GCP collection can be done using various options including ‘Manual Entry’, ‘Geocoded Images/Vectors’, ‘Chip Database’ or a ‘Text File’.

For THEOS rigorous model, a minimum of six accurate GCPs per image (or more, depending on the accuracy of the GCPs and accuracy requirements of the project) are required. After collecting the GCPs, select the ‘Model Calculation’ Processing Step and click on ‘Compute Model’ (see step 1.4). Check the ‘Residual Report’ panel (under the Reports processing step) to review the initial results.


4. Model Calculation
Once you have imported the file into the project, move to the Model Calculations step in the project and click on the Compute Model icon. Once the program has finished calculating the model, the image is ready to be orthorectified.


5. Generating Orthos
Proceed to the ‘Ortho Generation’ step and click on the ‘Schedule Ortho Generation’ icon. Select the file to be processed and select the appropriate DEM file. Set other processing parameters before generating the final ortho image.

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