Geodetic remote sensing

In remote sensing energy emanating from the earth’s surface is measured using a sensor mounted on an aircraft or spacecraft platform. The measurement is used to construct an image of the landscape beneath the platform (John et al., 2006).

Remote sensing is suitable for enormous areas (Richards, 2012). The advantage in the first place is to find the approximate course of the new artificial Chao Phraya River. The Digital Elevation Model (DEM) interpretes the earth’s surface and provides precise data from a large-scale area. Elevation data is one of the most essential aspects of assessing flood risk and for river designs and planning. It is necessary to use high terrain quality of DEM for calculating the average altitude, slope gravity, aspect, drainage, path simulation, etc. The flood extent and depth should be determined by the elevation information. For the new artificial river design, the first step of consideration should be the terrain surface of the research area.

6.2.1 Digital Elevation Model (DEM)

The Digital Elevation Model (DEM) is a digital model or 3D representation of a terrain's surface.

The topographic surface is arranged in a data file as a set of x,y,z coordinates where z equals the elevation. The comparative analysis of differences in ground, height, elevation and terrain are distinguished with two kind of DEM data sets; the Digital Surface model (DSM) and the Digital Terrain model (DTM). DTM represents the natural surface, i.e. the bare earth (Li et al., 2005).

This research uses these models for the new artificial river design and other benefits on topographic mapping, flood prevention, drainage mapping, river planning, ecological and forest preservation and infrastructure construction.

Figure 6.1 The difference between DSM and DTM in Festung Marienberg; Würzburg, Bavaria, Germany (LBDV, 2016)

Figure 6.2 Digital Terrain Model vs Digital Surface Model over Adelaide, South Australia (Aerometrex, 2011)

As online data, there are open source and public data sets of Digital Elevation Models (DEM) which are called the Shuttle Radar Topography Mission (SRTM) and the ASTER Global Digital Elevation Model ASTER GDEM:

Digital Surface Model

 DSM = Earth Surface including objects on it

 DTM = Earth Surface without any objects Digital Terrain Model

Vegetation

6.2.2 The general difference between SRTM and ASTER GDEM

1) Data from the Shuttle Radar Topography Mission (SRTM) is DEM data from NASA in 2009. The elevation models derived from the SRTM data are used in geographic information systems (GIS). The Shuttle Radar Topography Mission is an international project spearheaded by the U.S. National Geospatial-Intelligence Agency (NGA) and the U.S. National Aeronautics and Space Administration (NASA) (NASA, 2015).

2) The ASTER Global Digital Elevation Model (ASTER GDEM) is a joint product developed and made available to the public by the Ministry of Economy, Trade, and Industry (METI) of Japan and the United States National Aeronautics and Space Administration (NASA). It is generated from data collected from the Advanced Space Borne Thermal Emission and Reflection Radiometer (ASTER), a space borne earth observing optical instrument.

Presently, there are free available elevation data sets with sufficient resolution for coarse modeling: the Shuttle Radar Topography Mission (SRTM) DEM and ASTER DEM, with 90 m and 30 m resolution, respectively (METI, 2012).

Table 6.2 Character of data and the comparison between SRTM DEM and Aster GDEM (METI, 2012)

Information SRTM3 ASTER GDEM

Data source Space Shuttle Radar ASTER

Generation and

distribution NASA/USGS METI/NASA

Release year 2003~ 2009~

Posting interval 90m 30m

DEM accuracy

(stdev.) 10m 7-14m

DEM coverage 60 degrees north ~ 56

degrees south 83 degrees north ~ 83 degrees south

Area of missing data

Topographically steep area (due to radar characteristics)

Areas with no ASTER data due to constant cloud cover

(supplied by other DEM)

The SRTM dataset was generated using a synthetic aperture radar (SAR) system. Whilst the ASTER DEM seem the obvious choice because of its higher resolution, that's not the only control-point. ASTER DEM uses overlapping pairs of satellite images to estimate elevation, atmospheric interference, clouds, sun angle, and other variables which can introduce errors into the estimates (Ibid). Thus, for the water and flood management, the ASTER GDEM is more suitable because of the higher resolution and the higher accuracy.

In the following example, the ASTER and the SRTM image from the data that are available online are compared. In order to design the new artificial Chao Phraya River, the dataset of the elevation is the most important information. Currently, the accuracy that is available for free, is limited (Rees, 2014). In countries such as the US, the U.K and Germany mostly high resolution Terrain Models (DTMs) are already available, whereas in Thailand, it is still not yet possible. The following data compares the SRTM DEM and ASTER DEM on the area of Bangkok including Chao Phraya River (ibid).

Figure 6.3 Bangkok, ASTER 30 M Resolution (NASA, 2011)

Figure 6.4 Bangkok, SRTM 90 M Resolution (NASA, 2011)

The geographic characters of the area of Bangkok is flat and low-lying with an average of land elevation of just approximately 1.5 m above the sea level.

The figure 6.3 ASTER DEM on 30 m resolution illustrates that in the Chao Phraya River the elevation is higher than the city, which in reality is not possible. From Figure 6.4, the SRTM DEM 90 m resolution data illustrates that the river elevation seems to be more accurate in comparison with the ASTER DEM data.

The conclusion could be, that the NASA SRTM elevation can capture the water feature better than the data from ASTER. However, the ASTER data set has a three times better resolution as the SRTM. The data points that seem to be above the surrounding area along the Chao Phraya River line are possible due to the building and hotels which are located next to the Chao Phraya River.

In cooperation with international Digital Elevation Models (DEM), Germany and Japan already have high resolution Digital Elevation Models available on river basin scales. Even though DEM data of Thailand is available online, high-resolution data of the country is still difficult to achieve. Thai government office such as Land Development Department (LDD) or the Geo-Informatics and Space Technology Development Agency (Public Organization: GISDA) (GISDA, 2016) provide some data. In general, the elevation data is costly for science work.

This research receives a chance for German study supporting and testing with the DEM data from DLR.