class: title-slide, center, middle # GIS in Archaeology ## 09 - Terrain Data ### Martin Hinz #### Institut für Archäologische Wissenschaften, Universität Bern 27/11/24 .footnote[ .right[ .tiny[ You can download a [pdf of this presentation](gis_in_archaeology_09.pdf). ] ] ] --- ## Terrain .pull-left[   ] .pull-right[  ] .caption[ Photos by Martin Sattler, Matt Nelson, Pierpaolo Lanfrancotti on Unsplash ] --- ## Terrain .pull-left[ ### What is terrain > *An area of land, when considering its natural features. - Cambridge Dictionary* * Continually varying surface * What varies (for us) is elevation ### What is terrain analysis * calculating from the elevations and their spatial structure derived informations * Slope * Aspect * Curvature * Land forms * Cost Surfaces * Visibility * ... ] .pull-right[   .caption[ sources: Photo by Kasuma F. Gruber on Unsplash; https://digital-geography.com] ] --- ## DEM (Digital Elevation Model) .pull-left[ * DEM: a computer based representation of the terrain as elevation data * Mostly available as raster data, sometimes as TIN * large scale: Mostly from remote (satellite) data * small scale: areal photography or measurements, or even ground based surveys * methods * Radar * LiDAR * Structure from motion * ... ] .pull-right[  .caption[Sources: https://crisp.nus.edu.sg; https://desktop.arcgis.com] ] DEM can come in EPSG 4326 (WGS 84 lat/lng). Then might be necessary to reproject the DEM to a projected (meter based) CRS. We cover this in the next session... --- ## DEM Sources .pull-left[ * depends on your region and scale * GMTED2010 (https://topotools.cr.usgs.gov/gmted_viewer/viewer.htm) 7.5 arc second resolution (~ 225 m along the equator) * SRTM (e.g. http://dwtkns.com/srtm) 3 arc second resolution (~ 90 meters along the equator) * ASTER (e.g. https://search.earthdata.nasa.gov/) 3 arc second resolution (~ 30 meters along the equator) * TanDEM-X (90 m after registration, 12 m only with project submission) * LiDAR ] .pull-right[ .right[  ] .caption[Digital elevation model Bachu, China. Source: https://www.esa.int] ] --- ## Lets get some DEM * Go to http://dwtkns.com/srtm to get some SRTM data of a location of the world of your choice * Click on one of the tiles * Click on 'Download GeoTIFF' * Wait till it has finished and open it in QGIS * Check what CRS it comes in (remember?) .pull-right[ .right[  ] ] --- ## Reproject Raster .pull-left[ * the SRTM Tile comes in EPSG 4236 (WGS 84 Lat/Lng), which is degree based and unprojected * to work with the DEM, we have to reproject it to a meter based projection system * (Raster Reprojection always comes with smoothing/blurring data and loss of precision!) * which to choose depends on the location you have choosen to download * a good choice for projected CRS is Universal Transverse Mercator (UTM) * for the GIS Projection, you need to know * the number of the Zone you are in * if you are north or south of the Equator ] .pull-right[  ] --- ## Reproject to UTM using QGIS * I selected the SRTM tile around Switzerland * With that, I am at 32 N(orth) * Go to 'Raster > Projection > Transform (Reproject)' * Select your input Raster and CRS * Select your output CRS * search for in my case "utm 32N" * select the WGS 84 variant * click on 'Run' * Save your new Raster .center[   ] --- ## Slope .pull-left[ * Identifies the slope (gradient, or rate of maximum change in z-value) from each cell of a raster surface. * can be given in degree or in percentage > *Conceptually, the tool **fits a plane to the z-values of a 3 x 3 cell neighborhood** around the processing or center cell. The slope value of this plane is calculated ... The lower the slope value, the flatter the terrain - ArcGIS* ] .pull-right[   .caption[Source: https://desktop.arcgis.com] ] --- ## Calculating Slope in QGIS * Go to 'Raster > Analysis > Slope' * You could select to calculate percent instead of degree * usually you do not need to change anything * Click Run .center[   ] --- ## Calculating Slope Result * The resulting slopes range between 0 and 74° * You can change the symbology of the layer * a good choice might be the inverse spectral color ramp * archaeological significance: * flatter areas are better suited for building and agriculture .center[   ] --- ## Aspect .pull-left[ * The aspect of terrain refers to the direction it’s facing in * The pixels will have a value from 0-360° measured in degress from north indicating the azimuth * Flat areas having no downslope direction are given a value of 9999. > *Also here, the tool **fits a plane to the z-values of a 3 x 3 cell neighborhood** around the processing or center cell. Then the direction is calculated in which the plane is facing.* ] .pull-right[    .caption[Source: https://desktop.arcgis.com] ] --- ## Calculating Aspect in QGIS * Go to 'Raster > Analysis > Aspect' (Perspektive) * You could select to calculate the trigonometric angle (n, e, s, w) * usually you do not need to change anything * Click Run .center[   ] --- ## Calculating Aspect Result * The resulting angles range between 0 and 359.9999° * You can change the symbology of the layer * a good choice might be again the spectral color ramp * archaeological significance: * south-facing areas (on northern hemisphere) get more sun, maybe more likely for agriculture? .center[   ] --- ## TPI .pull-left[ * Topographic Position Index (TPI) is defined as the difference between the elevation at a cell and the average elevation in a cell that surrounds it within a predetermined radius (Weiss, 2001) * TPI values **above zero** show locations that are **higher** than the average, e.g. ridges * **negative TPI** values represent locations that are **lower** e.g. valleys * TPI **values near zero** are either **flat** areas **or** areas of **constant slope** ] .pull-right[  ] --- ## Calculating TPI in QGIS * Go to 'Raster > Analysis > Topographical Position (TPI)' * you actually can't change anything here * Click Run .center[   ] --- ## Calculating TPI Result * The resulting angles range between ~ +/- 350 m * You can change the symbology of the layer * a good choice might be again the spectral color ramp * archaeological significance: * ridges and peaks provide better control over areas: maybe preferred settlemen locations .center[   ] --- ## Landforms .pull-left[ * significance of the TPI depends strongly on size of the neighborhood * comining TPI from different neighborhood sizes reveals more 'natural' land form structures ] .pull-right[ .right[   ] ]  .caption[Source: http://www.jennessent.com] --- ## Calculating Landforms in QGIS * Not available from the menue * Open the Toolbox  * Start typing 'landform', find 'Tpi based landform classification' * Open the tool * select the correct layer * you can define the radii, 100 m does not make too much sense with our resolution... * you could also define a weighting according to distance and related parameters * **Do not now click on Run** ...this might take a while... .center[   ] --- ## Calculating Landforms Result .pull-left[ * The calculation might take quite a while * the result is a raster with up to 10 classes: * a good choice might be to [download this style](https://github.com/BernCoDALab/gia/raw/main/lectures/09/data/TPIbasedLandforms.qml) and use it * archaeological significance: * different classes of landscape attracts different usage ] .pull-right[ 1. Canyons, deeply incised streams 2. Midslope drainages, shallow valleys 3. Upland drainages, headwaters 4. U-shaped valleys 5. Plains 6. Open slopes 7. Upper slopes, mesas 8. Local ridges/hills in valleys 9. Midslope ridges, small hills in plains 10. Mountain tops, high ridges ] .center[   ] --- ## 'Basic Terrain Analysis' * for Landforms, we actually were using another GIS within QGIS: SAGA GIS * SAGA is a very good tool for morphometric (Terrain) Analysis * It offers eg. a small tool that achieve all of the above and much more. It is called 'Basic terrain analysis' * You might like to try it out .center[   ] --- # What We've Covered -- + Basics of Terrain Analysis -- + Slope -- + Aspect -- + TPI -- + Landforms --- ## More Terrain Analysis using SAGA .pull-left[ Olaya, V. (2004): A Gentle Introduction to SAGA GIS. http://downloads.sourceforge.net/saga-gis/SagaManual.pdf ] .pull-right[  ] --- ## Homework * Get the SRTM data from Ireland * calculate the TPI * send me a screen shot --- class: inverse, middle, center # Any questions?  .caption[Source: https://www.instagram.com/sadtopographies] .footnote[ .right[ .tiny[ You might find the course material (including the presentations) at https://github.com/BernCoDALab/gia You can see the rendered presentations at https://berncodalab.github.io/gia You can contact me at <a href="mailto:martin.hinz@unibe.ch">martin.hinz@unibe.ch</a> ] ] ]