GRASS, a GIS suitable for palaeoenvironmental...

GRASS, a GIS suitable for palaeoenvironmental studies


Yann Le Jeune, Physical geography laboratory of Meudon, CNRS research unit 8591,ValérieEmma Leroux, PhD student at the University of Rennes 1.

This lecture is from the user point of view.

Our aims: - a short description of the place of palaeoenvironmental studies in current archaeological issues;- the use of GIS (Geographical Information System) softwares in archaeology and palaeoenvironmental studies;- our use of the GRASS software in that context; several examples.


From treasure hunt to landscape archaeology

Item/Treasure

Building/Monument

Site/Village/Settlement

Cultivated areas

Environment

Landscape

Space

Evolution of archaeological issues


From treasure hunt to landscape archaeology

Item/Treasure

Building/Monument

Site/Village/Settlement

Cultivated areas

Environment

Landscape

Space

Evolution of archaeological issues

All these themas are now taken into account in archaeological issues through

Palaeoenvironmental studies


The use of GIS softwares in archaeologyGIS softwares are used by archaeologists as a georeferencing tool for various data (ancient maps, topographical maps, geological maps, etc.), in order to overlay and compare them.


The use of GIS softwares in archaeologyGIS softwares are used by archaeologists as a georeferencing tool for various data (ancient maps, topographical maps, geological maps, etc.), in order to overlay and compare them.

This kind of study requires mainly RASTER data.


GIS softwares are mainly used by archaeologists as a tool for documentary synthesis: management of databases of archaeological sites, comparison with ancient maps or other base maps, digitalization of paper maps, realization of thematic maps.

The use of GIS softwares in archaeology

This kind of study requires mainly VECTORIAL data overlayed on georeferenced pictures.

This tool is actually used as the digital version of paper atlases.

GIS softwares are sometimes used as a spatial analysis tool at a site scale: management of databases of artefacts and structures within the archaeological site, rare use of geostatistical analysis tools.


This kind of study requires mainly VECTORIAL data overlayed on georeferenced pictures (here the results of a geophysical survey)..


Mainly used to manage or visualize data, GIS softwares are less often used by archaeologists to create new data: modelling of the archaeological signal preservation, cost surface analysis, viewshed analysis, geostatistics related to known sites localisation.


This kind of modelling requires mainly RASTER data.

That is the kind of use that we would like to present.


The use of RASTER GIS softwares for modelling purpose is quite rare in French archaeology. We can try to find out some reasons for that:

French archeologists' education: mainly in archaeology/history/art history universities, with poor training in computer science except for office purpose; used to drawing or map editing, an archaeologist will be naturally fond of a VECTORIAL based software; RASTER based GIS are quite confidential compared to VECTORIAL based ones; inside the RASTER based softwares, the wellknown ones have heavy patent costs (ArcInfo, ArcView with the "spatial analyst" module, ErMapper) whereas the cheap ones (Idrisi, GRASS) remain confidential; besides, these last softwares are or were not translated in French until recently.

So... Why GRASS ? What is it ?



Our needs Better comprehension of past environments

Integration of all our various data (archaeological, geological, etc.)

+

Palaeoenvironmental modelling

Our use of GRASS

Accurate processing with Digital Elevation Model (DEM Modèle Numérique de Terrain, MNT in French terminology): the current relief is, in relative value, the only

environmental data which has not really changed during the last 10000 years.


Our use of GRASS

GRASS is clearly a tool for modelling. We use it more to create new data than to manage or visualize them (contrary to QGis for example).

Commonly referred to as GRASS, GRASS GIS is used for geospatial data management and analysis, image processing, graphics/maps production, spatial modeling and visualization. GRASS is currently used in academic and commercial settings around the world,as well as by many governmental agencies and environmental consulting companies.

http://grass.itc.it/


Our use of GRASS Example of GRASS interface


We have chosen GRASS some years ago for several reasons:

we need a RASTER based GIS to model various palaenvironmental data; GRASS is free; GRASS runs under Linux; that enables us to launch important modelling computations without the risk of crash and while being able to do other stuffs with our computer (multitasking): our computations sometimes require several days; GRASS is developed by researchers who work mainly in Physical geography; numerous integrated processes are thus directly usable for palaeoenvironmental studies: erosion, hydrology, morphoparameters; GRASS improves year after year and its GUI becomes more userfriendly; GRASS is a very educational tool to learn GIS: you can understand what you do, as each parameter is documented.

Our use of GRASS


Generation of environmental layers from the Digital Elevation Model:

Computations: height, slope, aspect.

Our use of GRASS / Examples





Integrated models: hydrologic network, morphoparameters (Woods Phd, 1998), watersheds.

Understanding archaeological spaces

archaeological sites






Digital Elevation Model of the area






DEM with altitude curves






Slopes






Reclassified aspect






Morphoparameters






Morphoparameters with a different resolution

SRTM are a free database downloadable on the NASA web site (Jet Propulsion laboratory). They cover the Earth except the poles with a resolution of 90m.

SRTM consisted of a specially modified radar system that flew onboard the Space Shuttle Endeavour during an 11day mission in February of 2000.



We use that kind of DEM when we work in places where such datas are too expensive (like ... in France) or too difficult to get (China).

GRASS directly imports SRTM DEM whith their exotic HGT format. A program (r.fill.null) corrects some SRTM errors, mostly on lake or snow surfaces.



Modelling of the evolution of forest areas:

digitalization of forest areas on georeferenced ancient maps,

computation of the age of forest areas (r.mapcalc).

Our use of GRASS / Ages of forest area







vers 1780






vers 1830






vers 1900






vers 1943






vers 2003






Age of forest areas


The RASTER format leads us to keep a close control on the data resolution: on that map, the resolution has been defined from the georeferencement error (of ancient maps), the produced maps embeds the data resolution (here 30 m).

Our use of GRASS / Raster resolution


Viewshed model around a site (r.los : line of sight) : we can test several heights for different hypotheses of buildings.

Our use of GRASS / Line of sight

Here, a fortified site and the question is: could that site control the valley above ?

Valley

Site





Valley

Site

Human height





Valley

Site

10 m high




The answer is no, even with a 20 m tower. The viewshed of the valley is obviously too small.


Valley

Site

20 m high

Using the r.los (line of sight) we created a simple script that browses the DEM and computates the surface viewed from each part of the map.

That cumulative visibility index gives us a measure of how hidden/visible places were (assuming an open landscape).

The resulting map is useful to understand the localisation of known sites or to look after new sites in particulary visible places.

This could be useful to understand past landscapes and placement of sites such as around old roads, funeral monuments, temples or megalithes, etc.

Our use of GRASS / Cumulative Visibility Index

#Région d'étude et résolutionpas="50"base_nord="2328500"base_est="156900"limite_nord="2334300"limite_est="163100"distance="5000"n=$base_norde=$base_est# Création de l'en tête du fichier de résultatdate > STAT_TOT.txtwhile [ $n le $limite_nord ]do

n=`expr $n + $pas`while [ $e le $limite_est ]do

...donee=$base_est

done


Our use of GRASS / Cumulative Visibility IndexExample of an ugly GRASS script

GRASS commands are in bold form.

g.remove rast=VUEe=`expr $e + $pas`# Calcul de la visibilité à partir du pointr.los input=MNT_IGN_carnoet output=VUE coordinate=$e,$n

obs_elev=1.75 max_dist=$distancer.mapcalc Surface_vue='if(VUE>0)'#Récupération des stats et transfert vers un fichier tampon inélégant mais

efficacer.stats input=Surface_vue fs=space nv=* nsteps=255 c n > STAT.txt# On attaque le fichier avec grep pour récupérer le nb de cellules vues au

point XY donnéSTAT=`grep "^1 " STAT.txt |cut d" " f2 `# Controleecho " Nb de cellules vues ="$STAT# On ajoute cela à un fichier texte de résultat (fichier de points) echo $e" "$n" "$STAT >> STAT_TOT.txt


# On crée un vecteur avec les résultats à partir du fichier de pointsg.remove vect=STAT_TOT v.in.ascii input=/home/yann/STAT_TOT.txt output=STAT_TOT format=point fs=" " x=1 y=2 z=0 cat=3# On transforme ça en raster pour avoir une vue plus claireg.remove rast=STAT_VISUAL_TOTv.to.rast input=STAT_TOT layer=1 output=STAT_VISUAL_TOT use=cat value=1 rows=4096# Affichage des métadonnéesr.info map=STAT_VISUAL_TOT





DEM




Cumulative visibility index

DEM




Cumulative visibility index reclassified


DEM




tombs field

medieval villages

unknown area

Cumulative visibility index reclassified


DEM


Our use of GRASS / Archaeological Signal ConservationWe combine some GRASS models (Woods' morphoparameters, Universal Soil Loss Equation, profile analysis) to modelize places where the archaeological site could be well preserved (Good ASC) and other places where the site could be more alterated by erosion processes.

erosion context (ASC )colluvial context (ASC +)no tendancyweak tendancies

We used NVIZ (3d GRASS viewer)to produce that image.

Open Source GISA GRASS GIS Approach

Markus Neteler and Helena Mitasova1. Edition 2002, 1st + 2nd reprint 2003, 3rd reprint 2004, 464 pagesISBN: 1402070888Kluwer Academic Publishers, Boston, DordrechtBook Series: The Kluwer international series in Engineering and Computer Science (SECS): Volume 689


References

GRASS Tutorial FR par Morritz Lennert: http://grass.itc.it/gdp/grass5tutor/HTML_fr/book1.html

GRASS in a nutshell FR: http://www.gdfhannover.de/lit_html/nutshell_v10_fr/index.html

ArcheOS – a Linux distribution for archaéology (and GIS, including GRASS...)http://www.arcteam.com/archeos/


References

http://grass.itc.it/gdp/grass5tutor/HTML_fr/book1.html

http://www.gdf-hannover.de/lit_html/nutshell_v10_fr/index.html


Thanks for your attention !

and thanks to free software developers ...

GRASS, a GIS suitable for palaeoenvironmental...

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