Comparison of Different Methods of Underground Survey

TS 5 – GNSS and Indoor Navigation

465

Comparison of Different Methods of Underground Survey Loris Redovniković1, Baldo Stančić1, Vlado Cetl1

1 Faculty of Geodesy, University of Zagreb, Kačićeva 26, Zagreb, Croatia, [email protected], [email protected], [email protected]

Abstract. The main goal of this paper is to investigate application possibilities of different instruments and surveying methods on underground objects. The Grapčeva cave on the island of Hvar in Croatia was selected as a test area. The cave was surveyed by polar and digital photogrammetric method. Total station, DistoX2 (Paperless Cave Surveying System) and digital still camera were the instruments used to conduct the survey. The obtained results of the data processing were analyzed, and the conclusions related to the possibilities of application, as well as the advantages and disadvantages of each method are presented.

Keywords: DistoX2, photogrammetry, total station, underground surveying.

1. Introduction

Very intensive development of technology provides surveyors a great variety of different instruments and methods for data collecting and processing. The survey of open space is greatly facilitated with GNSS. The surveyors are now facing the challenge to survey the areas where the use of GNSS is not applicable. The options for surveying of such areas are the subject of this paper. So far, there has been several papers published on this subject [Redovniković et al. 2014] and [Ballesteros et al. 2013], but there are still many questions to be answered. In order to achieve this goal, the additional research related to cave survey was conducted, and for this purpose, the Grapčeva cave on the island of Hvar was surveyed.

The Grapčeva cave is located on the southern slopes of the island of Hvar, near the hamlet of Humac. It is well known for its archaeological findings from the Late Neolithic (Hvar culture), Eneolithic period (ceramics of Nakovana and Ljubljansa culture, Adriatic type) to the Bronze Age (Illyrian pottery) [URL 1]. The cave is 25 m long and 32 m wide and located at the altitude of 223 m on hardly accessible terrain. In 1964, the cave was proclaimed a protected natural monument being ever since taken care of by the association "Humac". Various surveying methods were used for the cave survey on 15th and 16th of June 2015.

SIG 2016 – International Symposium on Engineering Geodesy, 20–22 May 2016, Varaždin, Croatia

466

2. Field survey

Three points of geodetic basis (G1, G2, and G3) were stabilized in the front of the cave entrance [Figure 3.1]. Their coordinates were determined by means of GNSS (Trimble R10) and CROPOS in the state HTRS96/TM coordinate system. Thus, the whole area of survey was positioned in space in absolute coordinates. Total station (Topcon Cygnus KS-102) was used for precise survey of the cave. It has declared angular measurement precision of 2” and distance measurement precision of ±(2 mm + 2 ppm) if measured with prism or ±(3 mm + 2 ppm) if measured without prism [URL 2].

The cave was also surveyed with DistoX2 system. It consists of hand laser distance meter (Leica X310) [URL 3]. It has declared distance measurement precision of ±2 mm. An electronic chip that enables the reading of azimuth angle and elevation angle with the precision of 0.5° [URL 4] has been installed in it. It has the possibility to store up to 1000 measured values. It can also send measured values by means of Bluetooth connection. The system can function as a stand-alone device, but its full functionality is the best when it connects via Bluetooth to an external (mobile) device (smartphone or tablet). There are a few free applications that can be used to receive and map measured data from DistoX2 in real time. For Windows OS, the most commonly used is the PocketTopo application [URL 5], while the application called TopoDroid is most often used for Android OS. In this research, the TopoDroid application was used. It was installed on a mobile device Samsung Galaxy Note 4, which enabled precise and simple mapping of the cave details in the real time.

The survey conducted with DistoX2 system resulted with 14 control points and 134 detail points. There were also 14 control points surveyed both with total station and with DistoX2 system. Out of these 14 control points, 8 of them were used as orientation points for photogrammetric survey [Figure 2.1].

Figure 2.1 Orientational points on a part of the Grapčeva cave


467

A part of the cave was surveyed by means of photogrammetric survey method using the digital still camera Canon PowerShot SX220 [URL 7].

The survey conducted with the total station resulted with 7 traverse points and 114 detail points.

The problem of insufficient lighting was encountered during photogrammetric survey. In order to eliminate the problem, two powerful caving lamps (Scurion) were used. There was also a problem with stabilization of orientation points. At the entrance area, it was possible to glue up the orientation points to the cave wall. However, it was an impossible mission in all other areas of the cave because of moister and dust on the walls. It was well known from earlier experience that the marks for orientation points must be plasticized after being printed in order to avoid their bending due to the impact of moisture.

3. Data processing and analysis of the results

The data collected by means of total station were processed by means of the software GeoMIR4 [URL 8], while the data collected with DistoX2 system were processed by means of the two most widely used caving software Therion [URL 9] and Compass [URL 10].

It is evident from [Figure 3.1] that the DistoX2 surveying system made it possible to survey some parts of the cave that could not be surveyed with total station.

Figure 3.1 Cave ground plan surveyed with DistoX2 system and mapped with Therion

software (blue) and the ground plan of the cave obtained with total station (red)


468

There is a very low north-western part of the cave where it was not possible to set up a total station. From the traverse point P7, through the small holes, two points were observed. From their position, the wall in the western part of the cave was visible that could not be completely surveyed from the traverse point P7. Since it was not possible to set up the instrument in that part of the cave, it was surveyed only with DistoX2 system.

During the surveying and data processing, special attention was paid to the part of the cave where the orientation points for photogrammetric survey were placed. The floor plan arrangement of the orientation points that were determined with the total station and DistoX2 system is shown in the following figure [Figure 3.2].

Figure 3.2 Variations of position for orientation points surveyed with total station and

DistoX2 system

The orientation points were observed from the traverse point P3. The coordinates obtained with DistoX2 system differed depending on the software that was used to process and depending on whether the alignment was carried out based on closing figures. Table 3.1 shows the values of the deviation between aligned coordinates supplied by DistoX2 system and the coordinates obtained with the total station. The average and standard deviations are also given.

Based on the analysis, Therion software proved to be a better choice compared to Compass. Therion provided the possibility to choose the state


469

coordinate system HTRS96/TM, which was not possible with Compass. In addition, Therion software can automatically generate .pdf files with top and profile view, while with Compass, we have to use some other tools for mapping. The disadvantage of Therion is that the computations are quite hidden from the user and they are not so intuitive. On the other hand, Compass has great a tool for searching blunders and a lot of statistical information about the cave. Both software offers the option of adjustments when it comes to closing figure or if we define fixed coordinates. Both software have also the possibility to produce 3D model of caves. Table 3.1 The differences of the coordinates obtained using a total station and DistoX2 system (m)

Total station – DistoX2 (Therion) Total station – DistoX2 (Compass) Pt. nr. dE dN dH dE dN dH Pt. nr.

15 0.14 0.19 -0.01 0.16 0.24 -0.01 15 30 0.10 0.17 0.00 0.12 0.21 -0.01 30 28 0.09 0.16 -0.01 0.11 0.21 -0.01 28 29 0.07 0.17 0.00 0.09 0.22 0.00 29 27 0.13 0.19 0.03 0.15 0.24 0.02 27 25 0.11 0.17 0.01 0.13 0.21 0.01 25 26 0.08 0.17 0.02 0.10 0.21 0.01 26 14 0.12 0.14 0.00 0.15 0.18 -0.01 14

Average 0.11 0.17 0.01 0.12 0.21 0.01 Average St. dev. 0.02 0.02 0.02 0.03 0.02 0.02 St. dev

The area that was measured with photogrammetric method was a vertical

wall of the cave, which is about 4 meters long and about 1.5 meters high. This part of the cave was used as an average detail that is prevalent in the cave. 39 photos were taken with different points of view to provide the best possible photo overlap. The point cloud is generated from photos using the software 3DSurvey [URL 11]. The generated point cloud contained about 3.7 million points [Figure 3.3].

Figure 3.3 Point cloud generated based on the coordinates of orientation points

obtained with total station


470

The aim of the research was to compare the accuracy of the point cloud that would be generated by means of the coordinates of orientation points obtained with DistoX2 system. The same photos were used for the production of one point cloud with the coordinates of orientation points obtained with total station and of the other point with the coordinates of orientation points obtained with DistoX2 system in Therion software. The comparison of the obtained point cloud was made by means of the free software CloudCompare [URL 12].

Two point clouds were overlapped on the basis of 900 000 common points. Statistical result of the deviations is given in the following table [Table 3.2]. Table 3.2 Statistical calculation of linear deviations between two point clouds given in (m)

Min dist. 0 Max dist. 0.233813Avg dist. 0.00772733Sigma 0.0179494Max error 0.0213441

Graphical representation gives a clearer picture of where the deviations occur

and in what amounts [Figure 3.4].

Figure 3.4 Linear deviations of the points clouds obtained with total station and with

DistoX2 system given in (m)


471

From the previous image [Figure 3.4] it is evident that the linear deviations between two point clouds are mostly less than 4 cm. For some small parts in the middle of the surveyed object, these deviations are between 5 and 10 cm, while the maximum deformation that amounts to approximately 20 cm can be found on the outskirts of the point cloud that are generally not the object of interest.

4. Conclusion

Based on the collected data and conducted analysis, we can conclude that the accuracy of the coordinates obtained with DistoX2 systems are quite satisfactory for the cave surveying needs. Of course, if we want to measure certain parts of the cave more precisely, it would be necessary to use a total station or a 3D laser scanner. The problem is that such equipment is not suitable for measuring the unfavourable parts such as narrow meanders and low or vertical parts of the cave. In addition, such equipment is quite expensive.

The advantages of DistoX2 system are that it is small, lightweight, portable and as such very suitable for measuring narrow, low and vertical space that is often found in caves. In addition to the advantage of mobility, it is also possible to map the details on the spot and in the real-time enabling higher quality mapping directly on the ground. It is possible to export data in a variety of specialized software tools that are generally free and offer various possibilities of finding blunders, adjustment, statistical calculations and many others.

Based on the representations of the analysis results, it was concluded that the application of photogrammetric methods for the determination of the 3D point cloud in the cave is possible. Due to poor lighting, the photographing takes relatively long time. The processing with a large number of photos can also take time. For these reasons, photogrammetric methods in this form are not suitable for surveying large cave systems, but it is suitable for imaging and 3D display of certain smaller part of the cave. These parts of the cave might be interesting to other researchers, such as geologists, archaeologists, palaeontologists, biologists and others, because of their special features. The point cloud thus obtained provides a variety of spatial information that can help in making conclusions to those who do not have the necessary knowledge and skills to navigate in the caves.

This research showed that the point cloud generated on the basis of the orientation points determined with DistoX2 system is very similar to the point cloud that is determined based on orientation points whose coordinates are obtained with total stations. Therefore, we can conclude that it is possible to use DistoX2 system to determine the coordinates of orientation points if we are satisfied with decimetre accuracy of a point cloud.

One of the following studies will examine the possibilities of applying the method of simultaneous localization and mapping (SLAM) for surveying caves.

Acknowledgments

We thank to Association ''Humac'' for the trust and support they have provided for us during this research.


472

This research was supported by the Croatian Science Foundation (HRZZ) under the number I-2485-2014; DEMLAS project.

References

Ballesteros, D.; Domínguez-Cuesta, M. J.; Jiménez-Sánchez, M.; González-Pumariega, P. (2013). Tape-compass-clinometer, DistoX or total station, what is the best method to elaborate a cave survey? A case study in El Pindal Cave, Spain, Poster, 8th International conference (AIG) on Geomorphology, 27-31 August, Paris.

Redovniković, L.; Ivković, M.; Cetl, V.; Sambunjak, I. (2014). Testing DistoX device for measuring in the unfavourable conditions, Proceedings of the 6th International Conference on Engineering Surveying, Kopáčik, A.; Kyrinovič, P.; Štroner, M. (ed.), Pregue: Czech Technical University, Faculty of Civil Engineering, pp. 269-274

URL 1: Croatia encyclopedia – network edition, ‘’Grapčeva špilja’’, http://www.enciklopedija.hr/Natuknica.aspx?ID=23140, (14. 3. 2016).

URL 2: 2LS Tools Cygnus Specifications, http://www.2ls-tools.com/en/products/cygnus/specifications, (14. 3. 2016).

URL 3: Manual Leica Disto X310, http://w3.leica-geosystems.com/downloads123/cp/disto/DISTO_X310/manuals/Leica%20DISTO%20X310%20MAN%20788479_en.pdf, (14. 3. 2016).

URL 4: 2LS DistoX2 User Manual, http://paperless.bheeb.ch/download/DistoX2_UserManual.pdf, (14. 3. 2016).

URL 5: PocketTopo User Manual, http://paperless.bheeb.ch/download/PocketTopoManual.pdf, (14. 3. 2016).

URL 6: TopoDroid User Manual, http://marcocorvi.altervista.org/caving/apps/tdmanual/index.htm, (14. 3. 2016).

URL 7: Canon Powershot SX220, http://www.ebay.com/itm/Canon-Powershot-SX220-HS-Grey-12-1MP-14x-Zoom-Digital-Camera-/220883054031, (14. 3. 2016).

URL 8: GeoMIR4, http://www.geomir.org/cms/index.php/download/category/4-geomir4, (14. 3. 2016).

URL 9: Therion, http://therion.speleo.sk/, (14. 3. 2016).

URL 10: Compass Cave Survey and Mapping Software, http://www.fountainware.com/compass/, (14. 3. 2016).


473

URL 11: Compass Cave Survey and Mapping Software, http://www.fountainware.com/compass/, (14. 3. 2016).

URL 12: CloudCompare, http://www.danielgm.net/cc/, (14. 3. 2016).

Usporedba različitih metoda izmjere podzemlja Sažetak. Cilj rada je istražiti mogućnost primjene različitih instrumenata i metoda mjerenja za izmjeru podzemnih objekata. Kao testno područje odabrana je Grapčeva špilja na otoku Hvaru. Špilja je izmjerena polarnom i digitalnom fotogrametrijskom metodom izmjere uz korištenje mjerne (totalne) stanice, DistoX2 (sustava za bespapirnu izmjeru špilja) i digitalnog fotoaparata. Nakon provedene izmjere i obrade podataka analizirani su dobiveni rezultati te doneseni zaključci u pogledu mogućnosti primjene, prednostima i nedostacima svake od metoda.

Ključne riječi: DistoX2, fotogrametrija, mjerna stanica, podzemna izmjera.

*scientific paper

Comparison of Different Methods of Underground Survey

Documents

Transcript of Comparison of Different Methods of Underground Survey