1.1. Historical and Archaeological Background
A large proportion of the Great Wall of China (GWC) that we see today was built during the Ming Dynasty (1368 A.D.–1644 A.D.) [
1]. However, west of the Yellow River in the Hexi Corridor, today’s Gansu Province (
Figure 1a) lies a section dating back to the Han Dynasty (206 B.C.–220 A.D.), more than 1300 years earlier than the Ming [
2,
3]. The Han Dynasty Great Wall (GH), the largest national defense project of the Han Empire, once spanned over 5000 km across the plains, grasslands, deserts, mountains, and rivers of northern China [
4,
5]. As one of the generations of the GWC, the GH was for centuries used to protect and safeguard the Han Empire against the invasions and attacks of the various nomadic powers of the Eurasian Steppes and Mongolia, who were looking to expand [
6,
7,
8]. Around 121 B.C., the Han Empire sent troops to fight against the Huns and re-conquered the lost territory of the Hexi Corridor [
9,
10]. Subsequently, in order to protect these recovered lands, the GH defense system was erected along the corridor over three periods of time (
Figure 1b).
These three periods of time were as follows. (1) In 121 B.C., Han troops regained the Hexi Corridor; thereafter, the GH was built between Lingju and Jiuquan (Lingju Section, yellow dotted line in
Figure 1b) [
11,
12]; (2) from 111 B.C. to 110 B.C., the GH was extended from Jiuquan to the Jade-Gate Pass in Dunhuang (Dunhuang Section, white dotted line in
Figure 1b) [
9,
13,
14]; and (3) from 104 B.C. to 100 B.C., the GH was again extended from the Jade-Gate Pass to Lop-Nor Lake to the west (Lop-Nor Section, blue dotted line in
Figure 1b) [
9,
14]. So, after twenty years of construction, the GH defense system in the Hexi Corridor was finally completed, starting from Lingju in the east and ending near Lop-Nor Lake in the west [
9,
14]. To consolidate the safety of the frontier region and to unblock the trade routes, the Han Empire built walls, forts and barriers along the Hexi Corridor to guarantee the safety and efficiency of trade activities and cultural communication along the ancient Silk Road [
15,
16]. Additionally, the defense capability of the GH was strengthened by the construction of beacon towers, passes, garrison stations, troop barracks, and also signaling capabilities that used flags and smoke in the daytime and fires at night [
17,
18,
19,
20].
Despite thousands of years of natural erosion and anthropogenic disturbances, there are still traces of the buried remains of these structures, located in different geographical environments. Over the past hundred years, the Dunhuang Section of the GH has been the focus of domestic and foreign studies in the fields of politics, archaeology, history, geography, agriculture, climate, the military and many other disciplines. Many explorers, geographers and archaeologists, such as Stein [
21], Hedin [
22], Xiang [
23], Xia [
24], Yan [
25] and Wu [
26], have made valuable contributions (
Figure 2) related to, for example, construction times, management structures and military functions. Comprehensive studies of the GH have been carried out by combining field investigations, archaeological excavations and literature analysis with textual research on the Han bamboo slips that have been unearthed [
14,
27,
28]. The achievements of these researchers provide an important and solid research background for the present study as well as useful archaeological and geographical materials.
The Dunhuang Section of the GH is an important physical carrier of and historical witness to the development of western China in ancient times. It has outstanding political, military, cultural, economic, historical, archaeological, geographical, ecological and environmental value and needs to be well studied and well protected. In terms of ecology, the areas along the GH in Dunhuang are mostly fragile and sensitive and are vulnerable to frequent environmental damage, which causes varying degrees of natural and anthropogenic damage to the GH. Therefore, it is particularly important to quickly and accurately understand the existing situation at the GH sites and to objectively evaluate and analyze them. This has great practical significance for the protection and sustainable development of the GH.
At present, the monitoring and protection of the large, linear GH site is mostly based on periodic ground patrols, especially in northwestern China, where the economy is less well developed and access to technology is limited. This has some obvious shortcomings: necessary action can be delayed, and this type of monitoring is time-consuming and costly. The conservation status of the GH is not good and faces severe challenges. At the same time, most of the GH remains are buried by sand or are located in remote areas. It is, thus, not easy to find these remains on the ground and making full use of VHR satellite RS images is necessary. This study is aiming to design an automatic approach for identifying the linear traces of GH, and provide a computer-based pattern recognition prototype which would be constantly advanced and used to monitor and evaluate the preserve situations of GH in the future.
1.2. Archaeological Remote Sensing
Remote sensing (RS) provides a rapid and low-cost way of exploring, mapping and monitoring archaeological features of interest (AOIs) across the world [
29]. Research that involves the identification of AOIs increasingly employs aerial photographs and spy satellite images [
30,
31,
32,
33] as well as multispectral and hyperspectral imagery [
34,
35,
36,
37,
38], SAR data [
39,
40,
41,
42], and LiDAR products [
43,
44,
45]. RS has unique advantages for detecting the large archaeological sites such as the Silk Road, Grand Canal, Nasca Lines and Great Wall [
4]. In the 1980s, sections of the Ming Dynasty Great Wall in Beijing and Ningxia were first surveyed using aerial remote sensing [
46,
47]. Guo [
48] discovered two generations of the Great Wall buried in dry sand in northwestern China by using Shuttle imaging radar data. Chen et al. [
49] completed the first comprehensive survey of the whole Ming Great Wall with the help of RS and GIS. Additionally, much RS-based research on the military defense systems of the Great Wall have been carried out [
50,
51,
52], especially at the regional level.
Nowadays, RS specialists and archaeologists are giving priority to manual visualization, which is limited to three spectral bands at a time [
29]. This means that it is still necessary to intervene manually, which requires a lot of time, manpower and material resources [
53,
54,
55]. With the rapid development of image and signal processing and computer vision, several (semi-) automatic approaches have also been designed for and applied to archaeological research [
56,
57,
58,
59,
60]. For instance, Lasaponara et al. [
55] designed a classification-based semi-automatic approach for identifying four buried farm objects at the Hierapolis site; the identification results were then qualitatively evaluated by combining visual interpretation with the GPR survey data. Traviglia and Torsello [
57] presented an integrated approach for automatically identifying archaeological landscape patterns based on multi-scale analysis of dominant oriented response filters, which qualitatively demonstrated that the proposed approach provided accurate localization of the target linear objects and alignments signaled by a wide range of physical entities with very different characteristics. Figorito and Tarantino [
58] identified artificial monuments indicative of crop-marks by combining VHR aerial photographs with an image segmentation approach; in this case, the performance of the proposed algorithm was qualitatively evaluated by classifying traces according to their visibility and integrity. Using a Circular Hough Transform (CHT) algorithm that had an average extraction accuracy better than 80%, Luo et al. [
59] automatically extracted the shafts of the ancient irrigation system of Qanat from VHR Google Earth imagery of a homogeneous desert environment. Only line tops of the Qanat shafts surrounded by spoil, representing line of tunnel gathering groundwater, can be observed from space, and their circular archaeological traces can also be seen in VHR Google Earth imagery [
59].
For RS specialists and archaeologists, obtaining spatial information about AOIs from various sources of data is the original and primary goal of archaeological remote sensing research [
59]. In general, AOIs can be detected in RS imagery as grid data or vector data using visual or automatic interpretation following image enhancement and data fusion [
61]. However, there are some long-standing problems pointed out by Luo et al. [
47], such as low level of automation and extremely limited range of spatial scales and spectral contrasts in AOIs’ identification. This study aimed to identify the linear traces of GH Linear traces from GF-1 MS and PAN data using an automatic approach that combined image segmentation with a Linear Hough Transform (LHT).
Section 2 presents the research materials, including a description of the study area at Dunhuang and the GF-1 satellite RS imagery.
Section 3 introduces the automatic approach applied to identify the linear traces of GH.
Section 4 and
Section 5 present the results and discussion, and the conclusions, respectively.