Research of Low‐altitude Photogrammetry Implementation Strategy of China's Ancient Circumvallation

www.as‐se.org/ssms Studies in Surveying and Mapping Science (SSMS) Volume 2, 2014

Research of Low‐altitude Photogrammetry Implementation Strategy of China's Ancient Circumvallation Project of Surveying and Mapping of Ming Dynasty Circumvallation (Southern Wall Section) Yu Yan*1, Zhe Li2, Haoran Shao3 National Key Lab of Surveying Tech Research for Architectural Heritages, Architecture School Tian Jin University No.92, Weijin road, Nankai District, Tianjin, China. *1

yanyu900606@126.com; 2lee_uav@tju.edu.cn; 3734800735@qq.com

Abstract: Most of the existing ancient circumvallation in China today is in the natural environment and the environment has never been cleaned up. As the blockings caused by the vegetation on the circumvallation itself, the surrounding vegetations, the constructions and other surface features affect the terrestrial laser scanning and the low altitude photogrammetry, there are often incomplete three‐dimensional points cloud data. Therefore, in the prior period of conservation works, surveying and mapping are difficult to implement under this circumstance.

In this paper, based on the surveying and mapping of ZhengDing Ming Circumvallation and some other ancient circumvallation in northern China, characteristics of Chinese northern ancient circumvallation are summarized in the terms of their structures and spatial arrangements as well as the distribution of natural obstacles, and section paradigm pictures protracted. On the basis of these pictures, distractors of surveying and mapping are classified and fine adjustments are made to each category in aspects such as multiple aerial baseline setting, Unmanned Aerial Vehicle(UAV) control and equipment selection, parameter adjustment, ground assistant work, and selection of season, time and weather. The purpose of such adjustments is to get the best possible integrated centimeter level point density surveying and mapping data of the circumvallation so that the goal of optimal application of the technique of UAV low‐altitude photogrammetry can be achieved.

Ancient circumvallation is the largest stock and most widespread type of architectural heritage in China. At present the surveying and mapping equipment available mainly falls into three categories: terrestrial laser scanning, low altitude photogrammetry and LIDAR. In this project, through analysis of characteristics, and summing‐up of rules, optimization of operating method is achieved, high density, high integrity surveying and mapping data under the condition of low cost and high efficiency is obtained.

The results of this experiment prove that low altitude photogrammetry is more suitable for the surveying and mapping requirements of China's ancient circumvallation at present.

This paper also gives a brief introduction to the LIDAR’s stronger penetrability and makes predictions about the application conditions, the effect and the promotion speed of ancient circumvallation’s surveying and mapping.

Keywords: Ancient circumvallation; Complex environment; UAV, Photogrammetry, Optimization

General Situation of Chinese Ancient Circumvallation Ancient circumvallation is the largest stock and most widespread type of architectural heritage in China. The circumvallation has a strong relationship with civilization development due to the preliminary function to provide protection and defense. As many dynasty‐changes (Through dynasties) accompanied by wars in China, a great amount of circumvallation has been built and remained.

In the modern society, circumvallation gradually changes to the tourist attraction and the reflection of history, and becomes a part of Chinese culture. However, circumvallation faces serious protection problems because of the aging of its internal structure, especially the loss of the rammed earth.  Roots of plant growing on the wall destroy its structure and introduce water into the wall, giving rise to the problems such as inner vacancy and collapse.  Long‐term weathering makes earth lose its viscosity. Earth gradually flows away. The whole wall becomes

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smaller and unstable and finally collapses.  People take away the earth for other uses. Climbing also peels down the earth from the surface.

FIG. 1 GENERAL SITUATION OF ZHENGDING MING CIRCUMVALLATION

Right now many circumvallation heritages, even in wild and complex surroundings, remain heavily‐damaged and obsolete, which makes it difficult to map by manpower or ground instrument (Fig. 1). Camera mapping by UAV also faces difficulties. In this paper, based on the analysis and practical surveying and mapping of the ancient circumvallation, we sum up the best working method to solve the above problems.

Normal Form of Chinese Ancient Circumvallation (Fig. 2) Normal Form of Wall Structure Circumvallation is composed of the wall body, the city gate, the gate tower and some other defensive utilities. It has some special structure characteristics in order to defend attacks.

It has a small tilting angle on the outside, nearly perpendicular to the ground, which is hard for enemies to climb.

While on the other hand, the slope on the inside is quite slow. This structure strengthens the stability.

Normal Form of Surroundings Main factors that affect mapping are plants and constructions around the relic. From the outside to the inside of the city are: trees on both banks of the moat (border trees), tall trees, plantations outside of the wall (both on top and at the bottom), plantations at the bottom of the inside, and buildings nearby.

FIG. 2 NORMAL FORM OF CHINESE ANCIENT CIRCUMVALLATION

Basic Mapping Pattern Circumvallation mapping requirement has three aspects. First, three dimension data of area within 30m on both sides should be complete; second, there must be detailed mapping data about the top and flank of the wall; moreover, the distance between three dimension points should be shorter than 3cm in order to show the subtle changes of the shape.

According to requirements above, mapping scheme is as follows. Use UAV to set up filming baselines on both sides and shoot at a tilted angle so as to easily generate point cloud of top, flank and ground. This method can guarantee a high data completion (Fig. 3). Besides, shooting distance should be around 40‐50m and resolution around 1cm, so point cloud can have a sufficient accuracy.

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According to real situation on site, shooting method can be adjusted slightly.

FIG. 3 TWO BASELINE SHOOTING PATTERN

Basic Points of Shooting in View of the Circumvallation Pattern The Choice of Lens and the Collocation of Routes The common lens focal length for Photogrammetry is in the range of 20mm to 50mm. In the project of surveying and mapping of ZhengDing Ming Circumvallation, we prepared a group of prime lens. Here we take 24mm and 36mm lens as example.

The resolution of the image about the circumvallation part is crucial to the precision of the point cloud. So we should fill the whole picture with the required parts, which are the top of the circumvallation, the wall metope and basis extension no matter which lens we choose. Thus, we make full use of the image and we can get high resolution as far as possible.

1) The Shooting of the South Side of the Circumvallation (Fig. 4) According to the aforementioned established environment of the circumvallation, there are two lines of 15 ~ 20‐metre tall border trees 20 meters away from the south of the circumvallation (Figure 2), and a plantation between the border trees and the circumvallation. Following disadvantages prevent us from choosing 24mm lens under this circumstance.

FIG. 4 COMPARISON BETWEEN 36MM AND 24MM LENS

 Because of the aforementioned principle of keeping the image with high resolution, we keep the UAV flying low and more close to the circumvallation so that the required shooting objects fill the image. Because the UAV is so close to the circumvallation without being above the forest plantations, the planting trees’ projection on the ground at the direction of the shooting angle is so large that it causes the blocking of the ground seriously, so it is hard to generate detailed point cloud of the ground in the software.  The angle between the shooting direction and the top of the circumvallation(α) is so small that it is not profitable for the generation of point cloud.  If we move the UAV closer to the circumvallation to increase the angle between the shooting direction and

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the top of the circumvallation, then the angle between the shooting direction and wall metope will decrease. So the 24 mm lens can't take into account of the shooting angle of every faces of the circumvallation.

2) 36mm lens will be more suitable under this circumstance.  When using 36mm lens, the altitude of the UAV just meet the minimum standards set by the height of the border trees (about 20 meters), even though sometimes we need to remote the UAV very close to the treetop to gain better shooting position.  In the actual shooting, the UAV is a little bit in the front, which is up front of the border trees. Thus, the UAV is just above the plantation. In the case of the UAV which has a certain height, through the settings of the optical axis of camera, we could make the shooting angle as far as possible through the branches to make the ground more exposure to the image.  This also makes the angle between the shooting direction and the plane of the top of the circumvallation (β) as well as the angle between the shooting direction and wall metope larger. The suitable shooting angle on each facet is conducive to the generation of a higher quality of point cloud.

3) The Shooting of the North Side of the Circumvallation Due to the increase of the population, the number of the multi‐storey residential building grows. Similar to the effect of border trees, modern multi‐storey residential buildings also become obstacles for photogrammetry. Thus, 36mm lens is suitable for the same reason.

According to the patter of the circumvallation, for the reasons of nature structure and acquired collapse, etc. The tilt angle of interior (north) wall is bigger than outer (south) wall. According to the basic principle of photogrammetry, which is setting the camera optical axis perpendicular to the plane of the objective to ensure the object distance variation range is small, we set the camera optical axis more pitched down while shooting the north side of the circumvallation(γ＞β)(Fig. 3).

4) Special Shooting Situation Foggy weather will affect clarity of the image, so 24mm lens should be used under such weather condition for closer shooting. This adjustment was adopted in mapping of Wengcheng, which is a part of Zhengding circumvallation.

Regional Processing for Sections Use special treatment on some shooting positions where it needs to draw sections, such as cleaning plants which blocks shooting from such positions. It is also possible to devise an independent airline around profile to do regional shooting. Just make sure to generate precise ribbon patterned point cloud.

Adding New Air Route

Two Baseline Shooting Adding Route Shooting

+

Final Point Cloud

=

FIG. 5 NEW ROUTE ADDING FIG. 6 OPTIMIZATION PROCESS

Because there are many plants blocking on the top of the circumvallation, using two normal baseline shooting method cannot acquire detailed point cloud. So another baseline should be added on the top of the circumvallation (Fig. 5). Shooting at the angle straight down can catch some images through the gaps between plants so better point cloud can be acquired in post processing (Fig. 6).

GPS plus Manpower Navigation Because of the wall’s curvature and high requirement for mapping accuracy, artificial operation is needed in addition

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to automotive flight based on GPS. Holders of higher quality should be used to keep the camera stable.

Other Strategies to Improve the Quality of Surveying and Mapping Results Season The chief climate is the temperate continental climate and temperate monsoon climate in the north of China, the main plants are temperate deciduous plants, so the deciduous season is the best season to do the photography measuring, it has the minimum interference caused by trees. The best advice is to do the same survey during the deciduous season.

The Photography Time Strategy Based on the Light According to the light changes in a day, we should select the different photography places and the photography time quantums of different wall orientations, for example, if the southwest section of outer wall orients side‐towards the southwest, it will be suitable to take a shoot in the afternoon on a sunny day.

On the sunny day without clouds, the strong lights to beam the south side of wall will produce many shadows in the north side, thus it will affect the generation of point clouds in the north side. In a word, we should choose the cloudy day to take photograph as much as possible, because the reflection of clouds will overcome the differences between the south and the north by making the lights more uniform.

Limitations To overcome the interference of surveying and mapping caused by the uncleaned natural environment around Chinese ancient city, we will implement flexibly as much as possible from these aspects: the ways of surveying and mapping have a great improvement (Fig.7). However, it still will meet some factors that affect the quality of achievements and cannot be conquered during the surveying and mapping, these factors are as follows:

FIG. 7 POINT CLOUD AFTER OPTIMIZATION FIG. 8 LIMITATIONS REFLECTED ON POINT CLOUD

Too Much Vegetation Under the existing condition, the lack of point clouds in the area of too much vegetation is inevitable (Fig. 8).

The Influence of the Haze Weather The point cloud generation and the sensory effects are seriously influenced by the weather condition, especially the air visibility. Under the condition of hazy weather, the quality of Yuecheng part was still affected even though we already tried to use a wide‐angle lens and to shoot closer during the field process to eliminate the weather interference as far as possible. In the contrast, south western lateral wall was shot under clear weather. Thus, the point cloud generated by that part was complete and color‐saturated. Therefore, more time should be reserved for this kind of surveying and mapping if possible for weather choosing.

The Exposure Difference between South and North of the Circumvallation.

The final results show that the colorful point cloud’s lightness of south and north side of circumvallation differ. The point clouds of different colors interlap and can be seen at the same time (Fig. 8). This situation is caused by the light difference of different sides of the circumvallation.

Automatically generation of accurate RGB data is one of the advantages of photogrammetry. Hence, we should look for automatic points polishing or cleaning method to eliminate this kind of color difference in the subsequent point cloud processing.

Future Expectations Compared with low altitude photogrammetry, LIDAR is less affected by vegetation blocking and can obtain more

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valid surface data in vegetation dense area, which is quite suitable for China's ancient circumvallation surveying area.

At the same time, LIDAR has the higher degree of automation compared with photogrammetry and can get rid of the interior work reducing the personnel operation error.

But at present the main application bottleneck of LIDAR is the high price of the equipment, high risk, and unit data error of airborne GPS and IMU, etc. The current measuring accuracy of LIDAR is only in the decimeter level.

In general, for earthen archaeological sites such as circumvallation and rampart, we should try to use LIDAR to improve the survey efficiency and to eliminate vegetation interference when the equipment gets more and more popular in the future.

Conclusion The surveying and mapping of the circumvallation sites is of great significance for the protection of the cultural treasure and for providing possibilities for people to acquire research and study history.

Through investigation and research, we summarize the pattern of ancient circumvallation in northern China. And through the practical project of low altitude photogrammetric surveying and mapping of Zhengding Ming Circumvallation, we prove that Vertical Take‐Off and Landing UAV is suitable for the surveying and mapping of the ancient city walls in a complex environment. According to the obstacles and various negative factors, we provide low‐altitude photogrammetry optimization strategy and experience for later surveying and mapping of such sites.

ACKNOWLEDGMENT

The work has been supported by the key research topic of State Administration of Cultural Heritage Key Research Base, “Low Altitude Remote Sensing Research of Architectural Heritage” (20120211) and also by NSFC Project (51108305).

REFERENCES

[1]

S.Wang. Origin and Development of Our Country’s Circumvallation. Shaanxi xi 'an: Relics and Museology, 2004: 1‐3.

[2]

Y.C.Yang. Disease Investigation and Research of Preservative Measure of Ancient Rammed Earth Wall in Qicheng.

Zhengzhou: Cultural Relics of Central China, 2008: 5‐6.

[3]

H.W.Cheng. Precise Surveying and Mapping of Ancient City Wall Based on 3d Laser Scanning Technology. Beijing: China Cultural Relics News, 2012.

[4]

G.Q.Yang. Cultural Significance and Contemporary Culture Protection of Circumvallation. Beijing: China Cultural Heritage, 2011: 3‐8.

[5]

Xue Kai, and Chen Wei. The Protection of the Vegetation Related the City Wall of the Ming Dynasty in Nanjing. Beijing: Architecture & Culture, 2010: 1‐4.

[6]

X.Y.Fu. Exploration of Chinese Ancient City Walls’ Protection. Beijing Forestry University, 2007(5): 26‐30.

[7]

Pan Qin. The Protection and Renovation of Jingzhou City Walls and Its Surroundings. HuaZhong University of Science and Technology, 2006(11): 23‐77.

[8]

Zheng Qi. Unveiling the Mysterious Veil of City Walls. Taking the Researches on Chinese Ancient City Walls for Example.

Wuhan City: Huazhong Architecture, 2012: 1‐4.

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