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Development techniques in soil survey
Soil survey is the study to search for the soil’s scientific data and information at any area to know the soil’s condition and characteristics, including the soil’s boundary, then making the map to show the soil’s existence, and interpreting the soil data for the purpose of utility as required and making a soil survey report.

        There are several levels of soil survey depending on the objectives to use the data; for example, the ‘reconnaissance soil survey’, the obtained map data would be for wide use, suitable for use in the national or regional scale. The ‘detailed soil survey’ would always be made in a small area; the obtained data would have very detailed description suitable for research and experimental work and soil and water conservation work for farm level. The reconnaissance soil survey takes a rather short time but can cover a wide area. The map used in the survey is a large-scale map, such as 1:250,000; each ‘check point’ or ‘boring’ (จุดตรวจสอบ) is very far from each other (such as one check point for every 10 sq km). While the ‘semi-detailed soil survey’ or ‘very detailed soil survey’ the number of check points per unit area is much greater; for example, 1 rai (0.4 acre) may have up to 10 check points. Therefore, the map produced for distribution or for any specific use must have a scale well harmonized with the purpose, such as the reconnaissance soil survey, the produced map will have a small scale, but for the semi-detailed soil survey, the produced map will have a large scale etc., whereas the user can bring the map to use according to the objectives correctly, properly and efficiently.

The soil survey and classification and soil map production in Thailand commenced from 1935 and have been developing continually. During the pioneering period, the surveyors had to travel by ox-carts, elephants, raft and on foot; that is why the drill to examine the soil characteristics in the field could be conducted only at specific places, and the search for the boundary of each soil type had to rely upon the surface rock map and the forestry map as the principle. Thereafter, there was aerial photography technology – the Aerial Photo Interpretation (API) – it was then applied through the stereoscope and it enabled us to see the shape of the terrain such as mountain, plain, creek rather concisely with the co-studying from the terrain maps to assist in the analysis to determine the area status boundary and to determine the boundary lines of the soil and the spots to make ‘exploratory boring’ (จุดเจาะสำรวจอย่างคร่าว). After that the surveyors would start to examine the soil characteristics in the real area to compare the data and adjust them as correct as reasonable.

 ( Three small figures )
Later on, various modern technologies were available, and at the same time the soil survey work needed to be done for more details and with certain specific issues, such as the project to improve the provincial soil map of 1:50,000 scale using the map unit according to 62 soil series groups. Such work besides displaying the data of soil boundary in the map, it also shows the soil suitability classes for the economic crop cultivation according to the kinds of crops that may produce the cost-effective economic rate of return in each province, so the satellite images were brought to assist in analyzing the terrain condition. The data obtained from the satellite image interpretation are the important principles in the examination of soil characteristics and land utilization in such vicinity, etc.
At present, there is an application of digital orthophotographs taken during 2002-2003 with adjusted coordinates, consisting of the Digital Elevation Model (DEM), contour lines, vertical and horizontal coordinates and the environmental factor data that are the soil forming factors: clorpt. These factors are used with the geographic information system (GIS) technology, remote sensing techniques, computer software to create 3-D model, hill shade and various basic data in digitization to interpret its meaning and to analyze the area condition to create the boundary of the map unit correctly in order to get closest to actuality.

The result from the application of the digital orthophotographs for survey and soil map production enabled the soil maps of Thailand to have high standard, close to those of the developed countries, both in the survey technique, classification and the control of the soil classification standard. The published soil maps that include the diagnosis of soil quality from the result of the soil survey and classification can be used for various purposes very well.

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