a photogrammetric method for calculating soil bulk density §

2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.plant-soil.com

496 DOI: 10.1002/jpln.201400010 J. Plant Nutr. Soil Sci. 2014, 177, 496–499

Short Communication

A photogrammetric method for calculating soil bulk density§

Thomas Bauer1*, Peter Strauss1, and Erwin Murer1

1 Institute for Land and Water Management Research, Federal Agency for Water Management, Pollnbergstraße 1, 3252 Petzenkirchen,Austria

AbstractSoil bulk density is an important parameter in many research fields. Unfortunately, no singlemethod exists for measuring bulk density for a wide range of soil conditions. We have carriedout tests and compared the results of a photogrammetric method for bulk density estimation asa supplement to widely-used methods (core method, excavation method).The differencesbetween the methods indicate the potential for using photogrammetry as a supplementary appli-cation for estimating soil bulk density.

Key words: density estimation / volumetric change / stone rich soil bulk density / coarse soil material

Accepted February 13, 2014

1 Introduction

Dry bulk density is a widely used parameter in research in thefield of soil science and agriculture (Reinsch and Grossman,1995; Timm et al., 2005). For example, this parameter isessential for evaluating the C stock and for soil hydrologymodeling (Neitsch et al., 2011; Sequeira et al., 2014; Suusteret al., 2011). However, in many conditions it is rather challen-ging to estimate the soil bulk density employing widely usedmethods (core sampling method, excavation method, clodmethod; ISO, 1998), or the results are not reliable (Nankoet al., 2014; Nemes et al., 2010). The main problems asso-ciated with these methods concern the reconstruction of sur-face conditions, inadequate sample size, or the use of themethods when measurements are performed in stone-richsoils. There is still no single method that can deal with all ofthese problems. To add to the discussion on improving themethodology for soil bulk density measurements, we testedthe potential of a method based on photogrammetry: (1) fordetermining volumetric changes and (2) for estimating thedensity of dry soil bulk.

The main advantage of photogrammetric methods is thatthey are easy to handle in the field, the observed area is notdisturbed (Warner, 1995), a permanent photogrammetricrecord is available, and high resolution of soil surfaces up tosub-millimeters can be achieved (Marzahn et al., 2012;Rieke-Zapp et al., 2001).

2 Material and Methods

2.1 Core sampling

Core sampling is the most widely used method for estimatingdry bulk density in soil science. For details on this method werefer to ISO 11272 (ISO, 1998). The main disadvantage ofthe core method is that it is only applicable for homogeneoussoils with little or no stone content.

2.2 Excavation method

For conditions in which core sampling is not suitable, e.g.,due to high stone content, the soil excavation method may beused. For details on this method we refer to ISO 11272 (ISO,1998). One of the main disadvantages of the excavationmethod is that uneven soil surface structures cannot behandled. The soil excavation method is therefore usable onlyfor flat soil surfaces.

2.3 Principles of the photogrammetric method

Photogrammetry is based on the principle of using multipleimages from different views to create 3D models of a surface.For our setting we used a calibrated DSR camera and a refer-

* Correspondence: T. Bauer; e-mail: [email protected]§ Manuscript based on the oral presentation by Murer, E., Bauer T.,and Strauss, P.: “Photogrammetrische Feldmethode zur Erfassungvon Oberflächenrauigkeit, Bodensetzung und Trockendichte”. http://eprints.dbges.de/883/1/Photogrammetie_Murer_V1.pdf DBG-Jah-restagung “Böden – Lebensgrundlage und Verantwortung”, Septem-ber, 7–12, 2013, Rostock, Germany.

ence frame (1 m × 1 m) with reference points to obtain infor-mation on scale and orientation (Fig. 1). Accuracy tests(Grims, 2013) showed that the method used by us results in3D models with precision in height and in position of < 1 mm.A minimum of 12 images of the soil surface (no direct sun-light) were recorded with a calibrated Nikon DSR camera.Table 1 gives camera specifications and a calibration report.A stereo image pair was taken from each side of the framefrom a height of approx. 1.5 m. Four additional images weretaken from each corner to establish a stable exterior orienta-tion. To produce point clouds from the images we used thecommercially available Photomodeler Scanner software (EosSystems Inc.). Image marking and matching is done automa-tically in this software, and we checked two quality para-meters (cf. models in Table 1). After checking the quality ofthe models, three scale distances and a zero point weremanually declared using the automatically identified refer-ence points. Then, the observation area was trimmed to 1 m× 1 m to reduce the calculation effort. Afterwards, four stereoimage pairs (one at every side of the reference frame) werechosen. Each pair had a b/h ratio between 0.2 and 0.6 and aconvergence angle of < 20°. However, the convergenceangle between the different stereo image pairs was muchhigher as every image pair was taken from another side ofthe reference frame. Out of all these stereo image pairs a sin-gle point cloud was produced (sampling interval 1 mm, pointreduction 25%). The resulting point cloud was exported as a.txt-file for further calculation. Further surface modeling andcalculations were done using self-developed data manipula-tion procedures within Matlab (The MathWorks TM). To deter-mine the volume of soil that was removed, surface modelswere calculated twice (command meshgrid), once before and

once after digging a hole. To fix the reference frame in thesame position while the pictures were being taken, we putbars into the soil and fixed them to the frame (Fig. 1). Figures2 to 4 present different surface models of the individual analy-sis steps on one plot. As in the excavation method, the ovendry weight of the removed soil has to be determined in orderto calculate the dry bulk density.

3 Comparison of methods and results

3.1 Volumetric tests

To test the potential of the photogrammetric method fordetecting volumetric changes, we added and removed var-ious amounts (+3 dm3 to –13 dm3) of sand and soil underlaboratory conditions to/from a surface and calculated thevolumetric changes. In total we performed 37 replicates forthis test.


Table 1: Description of the used equipment (camera type, resolution,and lens description), camera calibration protocol (true focal lengthand calibration values), and used model thresholds [overall and maxi-mum root mean square (RMS)].

Parameter Values

equipment

camera Nikkon D5100

resolution 4928 × 3264

lens (mm) AF-S DX Nikkor 18-55

fixed focal length (mm) 18

calibration

true focal length (mm) 18.957

K1 (mm) 2.40 × 10–4

K2 (mm) 2.33 × 10–8

P1 (mm) 6.38 × 10–6

P2 (mm) 4.37 × 10–5

overall RMS (pixels) 0.174

max. residual (pixels) 0.703

models

overall RMS (pixels) < 1.00

max. RMS (pixels) < 2.00

Figure 1: Reference frame and positioning on the bars with screwnuts.

Figure 2: 3D representation of undisturbed soil surface (in mm).

J. Plant Nutr. Soil Sci. 2014, 177, 496–499 Photogrammetric soil bulk density determination 497

Figure 5 compares the volumes photogrammetrically esti-mated with the amounts that were added and removed volu-metrically. A high coefficient of determination (r2 = 0.98) ofthe linear regression (y = –0.15 + 0.97x) indicates that volu-metric changes can be estimated accurately. The confidencelimits of the slope (CI95% are 0.93 and 1.02) indicate that theslope is not different from 1. The standard error of the esti-mate of the regression analysis is 0.49, indicating that thephotogrammetric received volumes are close to the volumesremoved or added.

3.2 Bulk density estimation

In the next step we performed the photogrammetric methodto estimate the bulk density at various field sites with little orno stone content (according to ISO 11272; ISO, 1998). Intotal we used 7 sites for calculation. Additionally to the photo-grammetric density estimation we took 9 core samples(200 cm3) from the first layer at each site.

Table 2 (left) compares the results of the core method andthe photogrammetric method. A mean difference between thecore method and the photogrammetric method of 0.09 g cm–3

was obtained. The difference was mainly due to the resultsfor site 2. At site 2, the soil surface was covered with a lot ofsmall cracks (< 2 cm in width). The cores were collected with-out cracks due to the small cylindrical volumes. In contrast,the photogrammetric method had cracks included due to thehigher sample volume. If this site is excluded from the calcu-lations, there is a much smaller mean difference (0.05 g cm–3)between the methods.

For sites which were rich in coarse material (10 to 40% ofmaterial > 2 mm), we applied the excavation method (accord-ing to ISO 11272; ISO, 1998), and again compared theresults with the photogrammetric method. We used the sameexcavation hole for both methods at each site.

This comparison (Table 2, right) between the excavationmethod and the photogrammetric method gave a mean differ-ence of 0.05 g cm–3. We could not observe any differences inthe accuracy of the photogrammetric method due to the size,form, and slopes of the holes made for bulk density estima-tion. The sand excavation method and the photogrammetricdevice can be compared because the volume calculation ismade for exactly the same hole. The mean sand excavationvolumes are 25 ± 193 cm3 larger than those for the photo-grammetric method. For an observed area of 1 m × 1 m, thiswill result in mean height differences of 0.03 ± 0.19 mm.

4 Conclusions

Tests on volume calculations indicate that the photogram-metric method is suitable for detecting changes in soilvolume. In addition, the method is comparable with widelyused methods like the core method and the excavationmethod. The main advantage of the photogrammetric methodis that the sampling size can be adjusted to site conditions.Hence, the soil bulk density can be estimated even for soilswith cracks or with a high stone content. The photogram-


Figure 3: 3D representation of soil surface after removing of material(in mm).

Figure 4: 3D representation of the resulting difference model (in mm).

Figure 5: Comparison of methods: photogrammetry and sand/soilreference (n = 37).

498 Bauer, Strauss, Murer J. Plant Nutr. Soil Sci. 2014, 177, 496–499

metric method is faster than the core method during samplecollection. The excavation method and the photogrammetricmethod are both easy to handle, and the time needed for car-rying out the analyses is approx. 30 min for both. Anotheradvantage of the photogrammetric method is its ability to cal-culate bulk densities even for rough surface conditions.

To further improve the method, we suggest including morevariation (i.e., more test sites). In addition, vertical bulk den-sity changes may affect the calculation and could be added.To obtain a good quality of the images, it is also necessary tofocus on light, camera, and surface conditions in order toobtain high precision surface models for shady study sites(e.g., forests). In addition to photogrammetry, other methodsthat produce high-resolution surface models, e.g., terrestriallaser scanners, could be tested under field conditions. Theymay not be as sensitive to light conditions as the proposedmethod. However, compared to this method costs for terres-trial laser scanning are much higher.

References

Grims, M. (2013): Semi-automatic assessment of soil surfaces withphotogrammetric methods. Diploma thesis, University of NaturalResources and Life Sciences, Vienna, Austria.

ISO (International Organization for Standardization) (1998): ISO11272. Soil quality–Determination of dry bulk density. ISO, Swit-zerland, pp. 10.

Marzahn, P., Rieke-Zapp, D., Ludwig, R. (2012): Assessment of soilsurface roughness statistics for microwave remote sensing appli-

cations using a simple photogrammetric acquisition system. J.Photogram. Remote Sens. 72, 80–89.

Nanko, K., Ugawa, S., Hashimoto, S., Imaya, A., Kobayashi, M.,Sakai, H., Ishizuka, S., Miura, S., Tanaka, N., Takahashi, M.,Kaneko, S. (2014): A pedotransfer function for estimating bulkdensity of forest soil in Japan affected by volcanic ash. Geoderma213, 36–45.

Neitsch, S. L., Arnold, J. G., Kiniry, J. R., Williams. J. R. (2011): Soiland Water Assessment Tool—Theoretical Documentation, Version2009. Texas Water Resources Institute, Texas, USA.

Nemes, A., Quebedeaux, B., Timlin, D. J. (2010): Ensembleapproach to provide uncertainty estimates of soil bulk density. SoilSci. Soc. Am. J. 74, 1938–1945.

Reinsch, T. G., Grossman, R. B. (1995): A method to predict bulkdensity of tilled Ap horizons. Soil Till. Res. 34, 95–104.

Rieke-Zapp, D., Wegmann, H., Santel, F., Nearing, M. (2001): Digitalphotogrammetry for measuring soil surface roughness. ASPRSAnnual Convention, St. Louis, USA, pp. 85–97.

Sequeira, C. H., Wills, S. A., Seybold, C. A., West, L. T. (2014):Predicting soil bulk density for incomplete databases. Geoderma213, 64–73.

Suuster, E., Ritz, C., Roostalu, H., Reintam, E., Kõlli, R., Astover, A.(2011): Soil bulk density pedotransfer functions of the humushorizon in arable soils. Geoderma 163, 74–82.

Timm, L. C., Pires, L. F., Reichardt, K., Roveratti, R., Oliveira, J. C.M., Bacchi, O. O. S. (2005): Soil bulk density evaluation byconventional and nuclear methods. Aust. J. Soil Res. 43, 97–103.

Warner, W. (1995): Mapping a three-dimensional soil surface withhand-held 35 mm photography. Soil Till. Res. 34, 187–197.


Table 2: Bulk densities of photogrammetric method (PM) and core method (CM) of 7 sites with low stone content, and photogrammetric methodand excavation method (EM) on 7 sites with high stone content.

Site No. PM CM (n = 9) Site No. PM EM

/ g cm–3 / g cm–3 / g cm–3 / g cm–3

1 1.36 1.43 ± 0.02 8 1.28 1.31

2 1.33 1.62 ± 0.01 9 1.90 1.99

3 1.57 1.56 ± 0.02 10 1.77 1.84

4 1.30 1.31 ± 0.02 11 1.45 1.54

5 1.24 1.33 ± 0.08 12 1.50 1.66

6 1.22 1.32 ± 0.04 13 1.66 1.57

7 1.28 1.32 ± 0.02 14 1.62 1.68

J. Plant Nutr. Soil Sci. 2014, 177, 496–499 Photogrammetric soil bulk density determination 499

a photogrammetric method for calculating soil bulk density §

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