Monitoring Islamic Archaeological Landscapes in Ethiopia Using Open Source Satellite Imagery

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The African landscape is set to change dramatically in the coming years, and will have a detrimental impact on the inherent archaeological and cultural heritage elements if not monitored adequately. This paper explores how satellite imagery, in
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  Full Terms & Conditions of access and use can be found athttps://www.tandfonline.com/action/journalInformation?journalCode=yjfa20  Journal of Field Archaeology ISSN: 0093-4690 (Print) 2042-4582 (Online) Journal homepage: https://www.tandfonline.com/loi/yjfa20 Monitoring Islamic Archaeological Landscapes inEthiopia Using Open Source Satellite Imagery Nadia Khalaf & Timothy Insoll To cite this article:  Nadia Khalaf & Timothy Insoll (2019): Monitoring Islamic ArchaeologicalLandscapes in Ethiopia Using Open Source Satellite Imagery, Journal of Field Archaeology, DOI:10.1080/00934690.2019.1629256 To link to this article: https://doi.org/10.1080/00934690.2019.1629256 © 2019 The Author(s). Published by InformaUK Limited, trading as Taylor & FrancisGroupPublished online: 09 Jul 2019.Submit your article to this journal View Crossmark data  Monitoring Islamic Archaeological Landscapes in Ethiopia Using Open SourceSatellite Imagery Nadia Khalaf and Timothy Insoll University of Exeter, Exeter, UK  ABSTRACT  The African landscape is set to change dramatically in the coming years, and will have a detrimentalimpact on the inherent archaeological and cultural heritage elements if not monitored adequately. This paper explores how satellite imagery, in particular open source imagery (Google Earth,multispectral satellite imagery from Landsat and Sentinel-2), can be utilized to monitor and protectsites that are already known with particular reference to Islamic archaeological sites in Ethiopia. Thefour sites used are in di ff  erent geographic and geomorphological areas: three on the SomaliPlateau (Harlaa, Harar, and Sheikh Hussein), and one on the edge of the Afar Depression (Nora),and have varied histories. The results indicate that open source satellite imagery o ff  ers amechanism for evaluating site status and conservation over time at a large scale, and can be usedon data from other areas of Africa by heritage professionals in the African continent at no cost. KEYWORDS Ethiopia; Islamic archaeology;Africa; satellite remotesensing; GIS Introduction: Remote Sensing in AfricanArchaeology The use of satellite remote sensing and aerial photography iscommon practice within many archaeological research pro- jects across the globe, generating important discoveries andhelping scholars understand the historical landscape. As sat-ellite technology improves and spatial and spectral capabili-ties advance, so does the ability to identify past humanactivity. While much of the current literature explores archae-ological site detection through satellite remote sensing (Campana 2003; Georgoula et al. 2004; Ricchetti 2004; Amin- zadeh and Samani 2006; Lasaponara and Masini 2006, 2007; De Laet, Paulissen, and Waelkens 2007, 2009; Masini and Lasaponara 2007; Grøn et al. 2011; Agapiou¸ et al. 2013), this paper explores how satellite imagery, in particular openaccess imagery, can be used to monitor and protect sitesthat are already known with particular reference to four Isla-mic archaeological sites in Ethiopia that are the focus of com-parative research being completed as part of the  “ Becoming Muslim ”  European Research Council Advanced Grantfunded project (694254 ERC-2015-AdG) in which Khalaf ispost-doctoral researcher and Insoll, principal investigator.This research project is assessing conversion to Islam andIslamization processes in eastern Ethiopia through archaeolo-gical excavation and survey at a range of sites, particularly Harar, Harlaa, and their surrounding regions. This is permit-ting evaluation of rural and urban Islam, pilgrimage andshrine-based practice, mosque architecture, the venerationof saints, and changes in lifeways through, for example,changes in diet, and the adoption of Arabic epigraphy andMuslim burial. Early and comparative evidence for the con-nection between Islam and long-distance trade, manifestedthrough the appearance of imported artifacts indicating par-ticipation in largely Muslim-dominated Red Sea and westernIndian Ocean networks, is also being explored (Insoll in press;Insoll and Zekaria in press).In general, very little research has been completed on Isla-mic archaeological sites in Ethiopia. The need to preserve andrecord the sites discussed here is therefore paramount, as solittle is known about Islamic archaeology in Ethiopia in com-parison to other aspects of later Iron Age or medieval archae-ology such as Christian or late-Classical sites (Insoll 2003,2017a; Phillipson 1998, 2009; Finneran 2007). The aspect of  the research described here has arisen from the need to remo-tely monitor the status of the sites investigated without con-stantly being in the  󿬁 eld, as changes to the sites have beenrapid even since the start of this research project in 2014.The current danger to archaeological sites in Africa, Isla-mic or otherwise, comes from an increase in urbanizationand infrastructure projects (Kankpeyeng and De Corse2004; Folorunso 2008; Bordes et al. 2008; Arazi 2009; Lane 2011). Environmental challenges are also a factor, including erosion,  󿬂 ooding, and deserti 󿬁 cation (MacEachern 2001;Arazi 2011; Marchant and Lane 2014; Lane 2015; Marchant et al. 2018), as is looting (Parcak et al. 2016; Fradley and Sheldrick  2017), which although not documented, is alsothe case at important sites in Ethiopia. There is also a threatto sites where con 󿬂 ict is lower level and less well-documented,as for example in Mali (Martinez 2015; Joy  2018). Remote sensing datasets and GIS databases have more recently beenused to monitor sites of historical importance, and havedocumented various human activities as threat to sites includ-ing agriculture, development (roads, railways, etc.), mineralextraction, and looting (Parcak  2007, 2017; Contreras and Brodie 2010; Brodie and Contreras 2012; Casana and Panahi- pour 2014; Agapiou et al. 2015; Nebbia et al. 2016; Parcak  et al. 2016; Casana and Laugier 2017; Rayne et al. 2017). Land use and land cover change studies using satellite ima-gery have documented the rapid increase in urbanism andinfrastructure throughout various countries in Africa (Yinet al. 2005; Angel et al. 2011; Forkour and Co 󿬁 e 2011; Mundiaand Murayama 2013; Debolini et al. 2015; Fenta et al. 2017). © 2019 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis GroupThis is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives License (http://creativecommons.org/licenses/by-nc-nd/4.0/ ),which permits non-commercial re-use, distribution, and reproduction in any medium, provided the srcinal work is properly cited, and is not altered, transformed, or built upon in any way. CONTACT  Nadia Khalaf  n.r.khalaf@exeter.ac.uk  Institute of Arab and Islamic Studies, University of Exeter, Stocker Road, Exeter, EX4 4ND, United Kingdom JOURNAL OF FIELD ARCHAEOLOGY https://doi.org/10.1080/00934690.2019.1629256  This rapid growth means that the archaeological record isunderthreatduetolossofinformationfromvariousconstruc-tion projects, expected to support the growth of populationacross Africa. Developing countries in Africa are projectedto have a greater urban population than rural by 2050, withmore than half of global population growth expected in thecontinent (Cohen 2006; Montgomery  2008; United Nations Department of Economic and Social A ff  airs 2017; UnitedNations Population Division 2017; United Nations WorldUrbanizations Prospects 2018). Rapid urban expansion indeveloping African countries is often unplanned (Tewoldand Cabral 2011; Mundia and Murayama 2013), which poses a great risk to existing and as-yet-undiscovered archae-ologicalsites.Thisdestructionispartiallymitigatedbysalvage/preventive archaeology; however, the extent to which this isimplemented varies signi 󿬁 cantly across Africa (Mcintosh1993; MacEachern 2001; Arazi 2009; Bedaux et al. 2011). In Ethiopia, preventative archaeology in advance of some majorconstruction projects is being completed. For example, theAddis Ababa to Djibouti railway saw archaeological survey completed on parts of the route in the Afar Rift Valley, andsimilar survey was undertaken ahead of the building of theGibe Dam (6°50 ’ 52.32 ” N, 37°18 ’ 6.78 ” E) in Ethiopia. However,the results are unpublished, and generally inaccessible (seeEthiopianElectricPowerCorporation[2010]foranexception).The African landscape is set to change dramatically in thecoming years, which is predicted to have a detrimental impacton the inherent archaeological and cultural heritage elementsif not monitored adequately. The potential of remote sensing to achieve e ff  ective monitoring has been highlighted by Mag-navita (2016, 116), who has described the bene 󿬁 ts of using satellite imagery, airborne survey, and ground-based geophy-sics in the African context.Satellite remote sensing in African archaeology has beengenerally used in areas where surface and sub-surface archae-ological remains can be seen on the ground either throughcrop marks or visible features; this approach often requireshigh-resolution satellite imagery. The use of high-resolutionsatellite imagery in particular is undoubtedly one of themost useful sources of aerial imagery data short of   󿬂 ying aplane or unmanned aerial vehicle over a selected study region(Lillesand, Kiefer, and Chipman 2015). Spatial resolution of an image is de 󿬁 ned by pixel size. High-resolution imagery is taken using satellite systems in orbit around the Earththat capture data at < 4 m per pixel, meaning that featureson the ground less than 4 m in size can be seen.Readily obtainable examples of high-resolution satellitedata are illustrated within the Google Earth application,although in many areas of the African continent, the imagery is not updated regularly. Raw high-resolution satellite ima-gery is available globally for download at a cost, which, forexample, was around $250 per 25 km 2 from Harris GeospatialSolutions (www.mapmart.com) in June 2018 with edu-cational discount and for existing rather than tasked imagery.These costs are prohibitive for most African heritage pro-fessionals, but the Digital Globe foundation (http://foundation.digitalglobe.com/) o ff  er grants for individuals atuniversity-level academic institutions for imagery and exper-tise for their research, which is a possibility for African scho-lars if they are eligible.Currently, there are few examples of satellite remote sen-sing and GIS being utilized in African archaeology (Katsamu-danga 2009; Sadr and Rodier 2012; Gaber et al. 2013; Harrower and D ’ Andrea 2014; Hunt and Sadr 2014; O ’ Regan,Wilkinson, and Marston 2016; Reid 2016; Sadr 2016), but it has been more widely applied in Saharan/North African con-texts where the archaeological remains can be easier to see(Moussa 2001; Mattingly and Sterry  2013; Merlo, Hakenbeck, and Balbo 2013; Shaltout and Ramzi 2014; Mattingly, Sterry, and Edwards 2015; Parcak et al. 2016; Biagetti et al. 2017; Rayne et al. 2017). A notable example of its application anduse in this area being provided by the EAMENA Project(Bewley et al. 2016).In contrast, there are many studies that show the bene 󿬁 tsof using satellite remote sensing for archaeological researchoutside the African continent (Beck et al. 2007; De Laet,Paulissen, and Waelkens 2007; Lasaponara and Masini2007; Garrison et al. 2008; Siart, Eitel, and Panagiotopoulos 2008). Most, if not all, of the case studies use high-resolutionimagery such as IKONOS and QuickBird, which have a highcost. The focus here is on four open access satellite imagery sources. First, the application Google Earth (and GoogleEarth Engine) is open access software available for downloadthat gives viewers a representation of the earth based on sat-ellite imagery. This software has boosted so-called  “ armchairarchaeology  ”  where anyone can use Google Earth to identify undiscovered archaeological sites (Australian Geographic2013). The other types of satellite imagery used come fromthree di ff  erent medium-resolution sensors; two of these sen-sors are currently in orbit (Landsat 8 and Sentinel-2) andone has been decommissioned (Landsat 5 TM). These med-ium-resolution sensors are not traditionally used for archae-ological research, because the spatial resolution is not greatenough to detect archaeological sites. However, these sensorsare used to analyze features of the landscape for accuratemapping (Gong et al. 2012), particularly urbanism (Li,Gong, and Liang  2015; Poursanidis, Chrysoulakisa, andMitrakaab 2015), cultivation and vegetation (Fan et al.2015; Schwieder et al. 2016), and detecting change in the landscape over time (Thomas et al. 2011; Son et al. 2015). These qualities can enable the identi 󿬁 cation of landscapedynamics that might be a threat to archaeology in the future. Study Area The four Islamic sites in Ethiopia discussed are Harlaa, Harar,Nora, and Sheikh Hussein. The rationale for their selection isthat they represent di ff  erent types of Islamic sites in Ethiopia.Harlaa is an abandoned partly-Muslim trading settlementconnected to the Somaliland coast and western IndianOcean trade networks. Harar is an extant Muslim city.Nora is an abandoned Muslim settlement connected withthe Sultanate of Ifat, and linked to the northern Tigray andDahlak Islands centered Red Sea trade networks. Finally,Sheikh Hussein is an important pilgrimage center attracting  visitors from across the region.The sites are in di ff  erent geographic and geomorphologicalareas, as indicated in Figure 1. Three are located on theSomali plateau. Harlaa is located at 1700 masl on the edgeof the main fault escarpment of the southern Afar margin,which is characterized by deeply incised terraces stretching more than 20 km (Pizzi et al. 2008; Coltorti et al. 2015) ( FIGURE  1). The landscape can be described as savannah,with scattered trees and grassland. Harlaa itself is heavily  vegetated with  Catha edulis  plantations. Harar is furthersouth in the Somali plateau, where there is less terracing  2 N. KHALAF AND T. INSOLL  Figure 1.  A) The site locations. (Produced by Khalaf.) B) Digital Elevation Model (Shuttle Radar Topography Mission [SRTM]) of the study areas with the main geo-morphological features labelled. Map adapted from Corti and colleagues (2015, 252), and produced by Khalaf. JOURNAL OF FIELD ARCHAEOLOGY 3  but more vegetation than in the Harlaa region. The elevationis ca. 1800 masl. Nora, like Harlaa, sits on the edge of the Afardepression, but on the central rim, on the opposite side fromHarlaa. The landscape has scattered trees, mainly following ephemeral  󿬂 uvial channels and sits at around 1100 masl.Sheikh Hussein is the farthest south. It is located at 1500masl and is in a dry zone with savanna type tree and grasscover (Agizew and Abegaz 2015). These sites, though theonly examples of Islamic sites archaeologically investigatedin Ethiopia, are representative of the types of threats facedby many other archaeological sites in Ethiopia, such as urban-ism (Harar), infrastructure development (Nora), populationgrowth and looting (Harlaa), and the impact of farming and environmental change (Sheikh Hussein).The impetus for using remote sensing as a monitoring toolat Harlaa stemmed from an extensive  󿬁 eld walking survey which indicated that signi 󿬁 cant areas of archaeologicalremains (ca. 6th/7th to 14th/15th centuries  A.D. ) had beendestroyed to obtain stone for reuse in modern building (Insollet al. 2017). As indicated in Figure 2a, borrow pits were fre- quently encountered with adjacent cairns formed of stoneblocks awaiting transport to building projects. To assess theextent of site destruction in the recent past, open access remo-tely sensed imagery was consulted. It was apparent that thisimagery was a very useful tool for monitoring the threats Har-laa faced and the study was extended to assess more generally the suitability of open access remote sensing datasets forinvestigating the conservation of the other three examplesof key Ethiopian Islamic sites. Some key issues were con-sidered. First, whether the datasets available were suitablefor assessing landscape change over time and if they couldbe used to pinpoint potential risk to the sites (anthropogenicand environmental landscape change). Second, if remote sen-sing was a cost-e ff  ective tool that could be implemented in theprofessional Ethiopian heritage context for site assessmentand monitoring. Harlaa (9°29  ’  10.22 ”  N, 41°54 ’  36.96  ”  E) Harlaa is located on the Harar road within the Dire DawaCity Administration area, ca. 15 km southwest of DireDawa, which is the fourth largest city in Ethiopia with a2007 census population  󿬁 gure of 233,224 (The World Bank 2007). The archaeological site is under and surrounding themodern Oromo village of Ganda Biyo. It is referred to hereas Harlaa, as this is the accepted name for the archaeologicalsite and is related to the common appellation of   “ Harla ”  givento ruined stone-built towns and funerary monuments, whosesrcins were ascribed by the Oromo to a legendary ancientpeople of giant status (Chekroun et al. 2011, 79), and whooccupied the region before the Oromo arrived (Joussaumeand Joussaume 1972, 22).Harlaa is archaeologically signi 󿬁 cant because it has pro- vided an archaeological sequence of up to 2.5 m depth thathas been AMS radiocarbon dated to between the 6th/7thand 14th/15th centuries  A.D.  (Insoll, MacLean, and Engda2016; Insoll et al. 2017). The site is formed of various com- ponents: settlement, cemeteries, mosques, and a defensivewall. Excavation has been completed between 2015 – 2018 infour areas, including a housing area, jewelry workshops, bur-ials, and a mosque (Insoll, MacLean, and Engda 2016; Insollet al. 2017). Extensive evidence for participation in bothregional and long-distance Red Sea and Indian Ocean tradenetworks, currently undergoing analyses, has also been recov-ered. The material includes, for example, glazed ceramics of likely Egyptian and Yemeni provenance; glass vessel and bra-celet fragments of probably similar srcin; glass and agatebeads, including examples visually analogous to Khambhat(Gujarat, western Indian) heat altered carnelian beads;cowry shells from the Indian Ocean and Red Sea; and ChineseQingbai and Celadon porcelain sherds (Insoll 2017b). Inter-national trade routes appear to have been directed towardZeyla on the Red Sea.Excessive  󿬂 ooding in the Dire Dawa area has been rela-tively well documented (Demessie 2007; Alemu 2009; Billi, Yonas, and Ciampalini 2015) and has also exacerbated ero-sion of various archaeological features in Harlaa, including burials, tomb complexes, pits, and sections of the defensivewall surrounding part of the site (Insoll 2018; Insoll,MacLean, and Engda 2016; Insoll et al. 2017). Besides extrac- tion of stone for construction, further damage to the archae-ology has been caused by constructing revetments for  󿬁 eldterracing. Building the  󿬁 eld terraces for growing subsistencecrops, primarily millet, and cash crops, particularly qat( Catha edulis ), has led to extensive landscape alteration asshown in Figure 2b, and has involved both removal and trun-cation of the archaeological deposits. Some localized looting of archaeological material has also occurred with ceramics,coins, and beads removed during farming activities, as wellas in more targeted form if objects such as coins are found. Harar (9°18 ’  33.21 ”  N, 42° 8 ’  15.84 ”  E) Harar is situated ca. 35 km southeast of Harlaa. It is the capi-tal of the Harari People ’ s National Regional State and in 2007had a population of 99,368 (The World Bank  2007). Harar islocated at 1900 masl, and has at its core a historic city, sur-rounded by a wall (the  djugel  ). This is built of locally quarriedcalcareous tu ff   (Hashi stone) mortared with mud and woodenreinforcements (Ahmed 1990, 321). It encompasses an area of ca. 1000 × 800 m and is accessed by   󿬁  ve gates, with a corre-sponding district of the city (Horton 1994, 195). The walledcity contains within it approximately 2000 houses, 82 mos-ques, and numerous saints ’  tombs and shrines (Interuniver-sity Research Center for Sustainable Development andHarari People National Regional State 2003), the resultof  important urban development over many centuries(Insoll 2017a).The walled city was given World Heritage status by UNESCO in 2006 (UNESCO n.d.,  “ World Heritage List ” ).Increasing urbanization is the primary threat to Harar andUNESCO recognizes that construction and infrastructuredevelopment pose potential threats to the conservation of the djugel, the city within, and the immediately surrounding area (UNESCO n.d.,  “ World Heritage List ” ). Excavations atnine locations in and immediately adjacent to the walledcity ( 󿬁  ve mosques, one shrine, two settlement areas, andone blacksmithing location) have provided AMS radiocarbondates between the 15th and 19th/early 20th centuries  A.D. (Insoll 2017a; Insoll and Zekaria in press). Harar functioned as a trade center that connected the east-ern Ethiopian highlands, arid lowlands, and the Gulf of Adenand it was also a center of Islamic learning and focal point of local shrine-based Muslim practice (Braukämper 2004; Insoll2017a). The city was capital of the sultanate of Adal (ca.1415to 1577), powerbase of Ahmad Gragn (b. 1506) in the  jihads 4 N. KHALAF AND T. INSOLL
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