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RTK GPS provides sufficient accuracy as it is possible to reach the centimeter level in the field with an average sky view above 50%. Using RTK GPS in an area with an average sky view above 55% can produce a good quality DTM, with elevation errors ranging from 0.3 cm to 6 cm, an absolute mean error of 2.44 cm, and a volumetric error of 0.5%. Applying RTK GPS to an area with an average sky view of less than 50% resulted in a low quality DTM where height errors are in decimeters and volumetric error is nearly 6%.

Bahan kajian ini adalah berdasarkan eksperimen untuk membandingkan aplikasi RTK GPS dengan ukuran TS. RTK GPS boleh memberikan ketepatan yang mencukupi kepada paras sentimeter kawasan darat dengan purata pandangan langit lebih daripada 50% lebih jelas. Aplikasi GPS RTK untuk kawasan dengan purata pandangan udara lebih daripada 55% boleh memberikan kualiti DTM yang baik dengan ralat ketinggian antara 0.3cm-6cm, ralat purata mutlak 2.44sm dan ralat volumetrik 0.5%.

Menggunakan GPS RTK pada kawasan dengan purata paparan langit kurang daripada 50% kejelasan menghasilkan DTM berkualiti renclah dengan ralat ketinggian desimeter dan ralat volumetrik hampir 6%.

INTRODUCTION

Background

Digital Terrain Model
GPS in OTM data collection

This technique is still used in many applications, especially for DTM generation and topographical mapping of medium to large areas [6]. At this spatial resolution, these data are commonly used to generate medium to low resolution DTM. The Airborne Laser Scanning (ALS), one of the members of the LTDAR technology, has been tested and proven to have an accuracy of several decimeters [I 0].

Land surveying using total station hCs has been the main tool for civil engineers and surveyors for DTM data C\acquisition as well as for outcrop surveying and large-scale mapping. Photogrammetry and remote sensing are commonly applied to the generation of medium to low-resolution DTM covering large areas. Certain filtering processes are required for DTM generation by photogrammetry, remote sensing and laser scanning as the direct derivative of that data is OEM or DSM which represents the earth with all its landscape features and land cover.

Kinematic GPS technique has been used for DTM data collection and can provide centimeter-level accuracy (7).

Problem Statement

To establish a high resolution DTM taking as the study area a p01iion of the Universiti Teknologi PETRONAS campus area. To assess the accuracy of RTK GPS as a tool for DTIVI data collection and to characterize the quality of the generated DTM, evaluating it against conventional land surveying technique using Total Station. The purpose of this work is to study the application of real-time Kinematic GPS for DTM data collection.

Grid-based sampling is performed to form the basis of the DTM quality measure. TS survey is used as a reference for the productivity and efficiency estimates as well as for the DTM quality measures. DTM quality measurements are performed by taking height error analysis together with volumetric error analysis.

The overview of the digital terrain model is given in the background section of this chapter, followed by a problem statement, objective, scope of the research and organization of the dissertation.

DIGITAL TERRAIN MODELING

Digital Terrain Model

A digital terrain model (DTM) is simply a statistical representation of a continuous ground surface with a large number of selected points with known X, Y, Z coordinates in a 1·bit coordinate field. Currently, the digital relief model refers to the representation of the so-called bare earth surface, without landscape features [7]. In addition to OTM, some other terms are used in terrain surface modeling such as Digital Elevation Model (OEM), Digital Surface Model (OSM), Digital Ground Model (OGM), Digital Elevation Model (OHM) and Digital Terrain Elevation Model (OTEM). . .

Lemmen [21] stated that DTM is OEM extended to include structural features such as drains, ridges, hilltops, depressions and other terrain discontinuities. Meanwhile, Wilson and Gallant [22] described DTM as OEM augmented by faultlines, where faultlines are lines in the topography where slope changes occur, such as the tops and toes of slopes. The term DSM generally refers to a representation of the Earth's surface, including landscape features such as vegetation and buildings [7].

Almost similar to the previous definition, Dowman et al [24) stated that the term Digital Surface Model (DSM) refers to an OEM, which represents the height of the first surface of a remote sensing system.

Figure 2.1 Illustration of break lines [23).

Digital Terrain Modeling

DTl\1 Data Collection Techniques

Aerial photogrammetry
Optical and radiometric remote sensing
Synthetic aperture radar
Triangle-based modeling
Grid-bllsed modeling
Hybrid modeling
Measures for DTIVl accuracy
Volumetric accuracy analysis

This phase difference, or the so-called input signal, can be used to derive the DTM of the imaged area [I 0]. The accuracy of using this technique to generate DTM depends on the terrain characteristics. The accuracy of the DTM generated by this method is highly dependent on the accuracy of the source map and the digitization process [6].

The accuracy of single frequency receivers is usually lower compared to dual frequency of the same manufacturer in question (3 8). According to Ley [39], there are four possible scores to evaluate the vertical accuracy of a DTM. Forecast based on production. (/Jrocedures): This is intended to estimate the likely errors introduced at the various production stages, along with the misjudgment of the vertical accuracy of the source materials.

This is cloned by defining a plane at a certain height of the DTM and calculating the corresponding volumes [5].

Summary of the Literature Review

Introduction

Orthometric height (H) is the height above an imaginary surface called the geoid, which is determined by Earth's gravity and approximated by mean sea level (MSL). The application of the DTM designed in this work is for large-scale projects, where it usually covers a relatively small area (radius less than two kilometers) [38]. Consequently, although the value of the geometric and orthometric height may be different, but the height difference is approximately the same.

In this work, both RTK GPS and TS use the same initial height provided by the existing benchmark at the Universiti Teknologi PETRONAS campus.

Study Area Description

DTM data collection by TS
Breaklines creation
Surface rendering
Volume computation

It is characterized by relatively flat terrain with a steep slope in the western part of the terrain. The images of the study areas (Terrain-I, Terrain-2 and Te1Tain 3) are shown in Figure 3.3. The specifications of the RTK GPS and the TS are available in Appendix A and Appendix B.

First, both studies were conducted during the same duration of data collection. This is especially to study the effect of obstacles on the accuracy of RTK GPS data and their influence on the quality of the generated DTM. The coordinates of the reference station (PGO 1) were used as starting coordinates to calculate the RTK positions of the rover throughout the study area.

The rover position is calculated based on the calculated baseline and the coordinates of the reference station. This included assigning the reference station coordinate and adjusting the RTK system properties. Meanwhile, the research on Terrain-2 and Terrain-3 aimed to study the effect of aerial visibility on the quality of RTK GPS data.

As shown in Figure 3.12, the center of the nail is at the intersection of the cross. This enables convenient instrument centering for both instruments (GPS pole and TS prism pole). In this work, the elevation error analysis was performed on all the study areas.

This is due to the fact that the quality of RTK GPS data usually depends on the view of the sky and the condition of the survey area. These are specifically related to the efficiency and productivity and accuracy of RTK GPS.

Experiment Workflow

RESULTS AND DISCUSSION

Obstruction Survey

Grid-based sampling

Vegetation on the northern and western edges of the area contributes the most to obstruction. This is a clue to the variety of vegetation that covers almost half of the total area. The result of the survey shows that fixed RTK solutions are relatively easy to achieve in an open area (sky view 75%).

Most of the adjustment solutions have approximately the same precision as obtained in the previous survey. As an example, the pond located on the eastern side of the study area was visualized. For this, it referred to the height of the detachment lines along the shore of the basin.

Slight differences between the actual and the model were noted in the rounded corners of the partition. The small ditch in the middle of the divider was ignored and no sampling points were collected. This was particularly the case on parts of the DTM where the sampling points were collected under an average sky visibility of 50%.

A worse error was discovered during part of the DTM, where the average sky visibility is 40%. DTM quality is highly dependent on the accuracy of the data source or input data. The comparison was performed by comparing the elevation value of the grid points given by RTK GPS with the value given by TS (complete data can be seen in Appendices D and E).

The summary of the statistical properties of the RTK GPS grid points in Terrain-2 is given in Table 4.5. Referring to the plot in Figure 4.19, it can be noted that the trend of the errors is correlated with the sky view level. Referring to table 4.6, it can be seen that some of the en·ors exceed the threshold.

The height errors of the respective grid points are significantly larger than those of the rest of the grid points.

Figure 4.3 Obstruction diagram of PGO I base station.

CONCLUSIONS AND RECOMMENDATIONS

Conclusions

The application of RTK GPS for data collection in areas with an average airspeed greater than 55% can provide good DTM quality, with elevation errors ranging from 0.3 cm to 6 cm. The absolute mean error of the respective DTM is 2.44 cm and the volumetric error is about 0.5%. For this, the height errors, as well as the absolute mean error, are at the centimeter level, while the volumetric error is approximately 1%.

Application of the RTK GPS data acquisition tool in an area with less than 50% sky visibility is not recommended as the resulting DTM may be unreliable. The experimental result showed that it reached the decimeter level (16.685 cm) of height error and almost 6% of volume error.

Recommendations and Future Works

7] Nico, G., et al., Generation of digital terrain models with a synthetic aperture ground-based radar interferometer, lEEE Transactions on Geoscience and Remote Sensing, Vol.43 No.I, p. 21] Lemmens, M.J.P.M., Uncertainty in Automatically Sampled Digital Elevation Models, Spatial Accuracy Assessment - Land Information Uncertainty in Natural Resources, CRC Press, Washington, 2000. 34] Teunissen, P.J.G, Least Squares Ambiguity Decorrelation Adjustment: A Method for Rapid Integer Estimation GPS ambiguities, Journal of Geodesy, Vol.