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Digital terrain modelling in archaeology : three-dimensional GIS, examples

from Birka and Vendel

Persson, Kjell

Fornvännen 2002(97):3, s. [191]-196

http://kulturarvsdata.se/raa/fornvannen/html/2002_191

Ingår i: samla.raa.se

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Digital Terrain Modelling in Archaeology

Three-dimensional GIS, examples from Birka and Vendel

By Kjell Persson

Persson, K. 2002. Digital Terrain Modelling in Archaeology. Three-dimensional GIS. Examples from Birka and Vendel. (Tredimensionell GIS.Digitala terräng-modeller i arkeologin. Exempel från Birka och Vendel. ) Fornvännen gy. Stock-holm.

Digital terrain modelling (DTM, 3D GIS) is briefly described and two case stu-dies, from Birka and Vendel, presented. DTM methodology allows the combi-nation of different spatial data categories in a three-dimensional terrain model. The case studies show how landscapes of the past thal have been dramatically al-tered—a! Birka by shore displacement, at Vendel by recent landscaping—can be reconstructed and visualised wilh lhe aid of a computer.

Kjell Persson, Archaeological Research Laborators; Stockholm University, SE-106 g i Storkholm, Sweden.

Kjell.Persson@arklab.su.se

Urban and regional planners sometimes use digital three-dimensional terrain models to combine topographic and other information in a Geographical Information System (GIS). Complementary information about e.g. geology, predpitation, cultivation and road networks can be added to these models. Areas for pos-sible development can then be analysed and evaluated from their accessibility, suitability or exposure. T h e method is suitable for visually si-mulating e.g. the effects of damming for kyd-roelectric power or traffic noise protection.

DTM methodology (Digital Terrain Model-ling) provides a better basis for decisions in so-cial planning as it allows the combination of different spatial data categories in a three-di-mensional terrain model. It is an important tool in environmental consequences studies. T h e method makes it possible to test compre-hensibly and visually analyse different combi-nations of data. The result is available in tke computer immediately after analysis and can at

low cost be presented in printed reports or directly from tke computer in slide skows (Aro-noff 1991, Bonkam 1994, Malmström 1995).

In arckaeology, two-dimensional GIS meth-odology bas been used, but archaeologists also need methods to combine different spatial da-ta applied to three-dimensional terrain m o dels. DTM is especially applicable in investiga-tions of slope activities, such as building terra-ces, shore-connected activities, or in regional analyses of landscape changes caused by shore displacement

Furthermore the method is well suited to reconstrueting the three-dimensional terrain of an archaeological site even if it has been ex-cavated and then covered by buildings or changed in some other way. If early maps with topographic information are available it is pos-sible to digi tally collect a large n u m b e r of coor-dinates and build a database with the original XYZ data. In exeavation reports one sometimes finds contour maps drawn on the basis of a

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sur-192 Kjell Persson

Fig. 1. Three-dimensional model of lhe island of Björkö. Gontour 5 m a.s.L. emphasised. modell av Björkö. Femmetersnivän över dagens strandlinje markerad.

-Tredimensionell

vey with a levelling instrument. These maps of-ten use local coordinate systems, but if a few still existing points can be identified on the map, one can convert the data to a generally used coordinate system. Then the model can be combined with new spatial information gathe-red from surveying, prospecting or exeavation.

At the Archaeological Research Laboratory of Stockholm University we have for many years tested different CAD (Computer Aided Design) and GIS programs and tried to adapt tkem to our archaeological needs. Above all we have tried to develop methods meant not only to visually pre-sent the results from our excavations but also to be used interactively as interpretational tools during excavations. For ten years, our la-boratory has successfully worked with two-di-mensional GIS, and latdy also with three-di-mensional modelling teckniques.

From databases witk XYZ coordinates, vec-tor based systems can construet triangulated tkree-dimensional m o d d s of tke terrain. Coor-dinates are collected witk total stations or GPS

(Global Positioning System) or digitised from contour maps. Often several data sources are combined. By statistic interpolation the digiti-sed coordinates can be udigiti-sed to calculate new coordinates and hence increase tke accuracy of tke triangulated surface.

Shoreline studies at Björkö

We kave used tkis technique to study skore

displacement at tke island of Björkö in Lake Mälaren, Middle Sweden, in an attempt to find suitable locations for harbours or landing spöts for boats connected to the garrison of Birka

(Stålberg 2000). From a topographic map with five-meter equidistance we digitised a n u m b e r of coordinates along each elevation contour. The mapping program SURFER was used to in-terpolate a great number of new points tkrougk tke Kriging metkod (Cressie 1990), and from tke digitised and interpolated coordinates a tkree-dimensional model of Birka was eonstrueted

(fig. 1). Tke shoreline of the Viking Period is calculated to have been situated about 4—5 me-ters above the present one, and the new map shows the island to have been divided in two at the time. By rotating the model and possibly in-creasing the height scale we can study these islands from different views to search for sui-table locations at which to land boats. It is also possible to measure volumes.

To furtker study tke effects we imported tke database into the GAD program MicroStation for handling in 3D. Here we can apply a colou-red and textucolou-red "skin" to the triangulated sur-face, simulating, for instance, grass. T h e ligh-ting can be varied to increase the visibility of se-lected structures. A simulated, movable water-table can be added to study the effects of diffe-rent sea levels (figs. 2—3). We can also measure length and areas. Finally, we have access to all the CAD program's drawing facilities in three

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Digital Eerrain Modelling in Archaeohgy 1 9 3

Fig. 2. Model of present-day Björkö. — Modell av dagens Björkö.

Fig. 3. Model of Björkö in the Viking Period. — Modell av vikingatidens Björkö.

dimensions, and information about known struc-tures such as the town rampart, under-water palisades, cemeteries and exeavation trenches can be added to the surface.

Reconstruction of a house terrace in Vendel We have also used the DTM technique within the SIV (Svealand in the Vendel and Viking Periods) project, which is a co-operation be-tween the Archaeological Research Laboratory and the Department of Archaeology and An-cient History at Uppsala University. Compre-hensive geophysical and geochemical prospec-ting in Vendel, central Sweden, indicated a pos-sible settlement on the slope just south of the church and near the famous boat graves (Persson 1998).

Surveying with GPR (Ground Penetrating Radar) along a line down lhe southern slope of the ridge revealed a horiz.ontal u n d e r g r o u n d reflection, which was interpreted as a förmer terrace surface, covered by låter sedimenta-tion. Excavations confirmed this interpreta-tion, and 011 the terrace we found a settlement witk kotise remains contemporary witk the boat graves (Isaksson 1997).

In tbc 1960.S the southern slope landscaped to a smooth lawn and the terrace is not visible today. In 1930, however, professor Nils Äberg mapped the area when searching for further boat graves, and the terrace is visible on his contour map (fig 4 ) .

By replacing the topographic information from the present map with Äberg's resuhs

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1 9 4 Kjell Persson .." i .

k „ „

3 t i vcuocu xyttK* S, "1 KyKWI OUK W T / j e UpOä&tjOr" » » " •

Fig 4. Topographic map showing the area south of Vendel church measured by Nils Äberg 1930. — Topografisk karta över området söder om Vendels kyrka upprättad av Nils Äberg i g s o .

Fig. 5. Reconstruction of the topography at Vendel church according to Äberg's survey of 1 g30, with the sub-sequently destroyed prehistoric terrace at the arrow. — Rekonstruktion av topografin vid Vendels kyrka utifrån Äbergs kartering från ig3Q, med den sedermera förstörda förhistoriska terrassen vid pilen.

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Digital Terrain Modelling in Archaeology 1 9 5 Fig. 6. T h e Late Iron Age house at

Vendel church with excavated postholes and wall-line emphasi-sed. Reconstruction in thin lines. — Yngrejärnåldershuset vid Ven-dels kyrka. Stolphål och vägglinje markerad. Rekonstruktion i tun-na linjer. O O cP O9 O O

f o

O CD

o°°

o° O

o° °

0

o

o o

o o o

o

Fig. 7. T h e Late Iron Age house at Vendel church, reconstruction of posts and supporting stone line. — Yngre-järnåldershuset vid Vendels kyrka. Rekonstruktion av stolpar och syllstensrad.

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196 Kjell Persson

south of the church we have eonstrueted a to-pographic map of the ridge at Vendel church as it looked in 1930. From that map we have di-gitised a n u m b e r of points along the different elevation contours and built a database of XYZ coordinates. After interpolation in SUREER we eonstrueted a three-dimensional model of the terrain as it looked before the lawn was laid out ( %

5)-T k e database with both tke digitised and tke statistically interpolated coordinates was tken imported into MicroStation and a terrain surface was triangulated. Tkis surface can now be edited in tke CAD program. It is possible to to "excavate digitally" by drawing the trenches and the profiles. We have added information from the exeavation plans witk postholes and a stone wall at the terrace foot. A gravel pit from the time of tke building of tke church in tke 1 3tk century kad destroyed tke deposits at tke upper part of tke kouse. Only tke postkoles of tke two soutkern gable posts and seven roof supporting posts were preserved (fig. 6). It was thus not possible to determine the length of the house, but from Äberg's information we can see that altogetker five pairs of roof suppor-ting posts and tke four gable-posts would have fitted in on the terrace, and that the length of the house would have been about thirty meters at the most.

In the CAD program one can dig postkoles and erect posts which makes it possible to visu-ally test the result and reconstruct the house. O n e can also add all available information.

background, texture and shading, to get a rea-listic model of the exeavation results. In our model you can see the posts of the reconstruc-ted house, some posts from another building and the supporting stone wall at the terrace foot (fig. 7). Vendel church now occupies the top of the ridge but there are indications of another house terrace there.

DTM methodology provides a realistic three-dimensional picture of our interpretation of the exeavation. It is of great pedagogical value to be able to look at tke site from different ang-les and, especially, to measure lengtk and are-as. We have found this methodology very use-ful and we hope that others will do likewise. References

Aronoff, S. i gg 1. Gcographic Information Systems: A Ma-nagement Perspective. Second edition. Ottawa. Bonham-Garter, G.F. igg4- Gcographic Information

Sys-lems for Geosdentisls. Modetting with GIS. Oxford. Cressie, N.A.C. iggo. The Origins of Kriging.

Mathe-matical Geology V 22, pp. 239-252.

Isaksson, S. 1 gg7- Arkeologiska boplalsundersökning-ar vid Vendels kyrka igg6. Arrhenius, B. & Erikson, G. SIV. Svealand i Vendel och Vikingatid. Raf/portfrän utgrävningarna i Vendel. Stockholm. Malmslröm, B. 8c Wellving, A. igg5. Introduktion till

GIS. Gävle.

Persson, K. igg8. Archaeological Prospection. Labo-raliv Arkeologi 10/11 pp. 17-26. Stockholm. Stålberg, K. 2000. Hade Garnisonen en hamn ?

I*rnspek-teringav strandområdet nedanför Garnisonen i Birka. C/D-uppsats i laboraliv arkeologi gg/00. Stock-holms Universitet.

Sammanfattning

Digitala terrängmodeller (DTM, 3D-GIS) be-skrivs kortfattat och två fallstudier, från Birka och Vendel, presenteras. DTM-metodologin gör det möjligt att kombinera olika slags rums-liga data i en tredimensionell terrängmodell. Fallstudierna visar hur forntida landskap som

har genomgått drastiska förändringar - vid Birka på g r u n d av strandförskjutningen, vid Vendel på g r u n d av sentida omsckaktning -kan rekonstrueras ock visualiseras med datorns hjälp.

Figure

Fig. 1. Three-dimensional model of lhe island of Björkö. Gontour 5 m a.s.L. emphasised
Fig. 2. Model of present-day Björkö. — Modell av dagens Björkö.
Fig 4. Topographic map showing the area south of Vendel church measured by Nils Äberg 1930
Fig. 7.  T h e Late Iron Age house at Vendel church, reconstruction of posts and supporting stone line

References

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