GIS in Archaeology

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#  GIS in Archaeology

##  09 - Terrain Data

###  Martin Hinz

####  Institut für Archäologische Wissenschaften, Universität Bern

27/11/24
.footnote[
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You can download a [pdf of this presentation](gis_in_archaeology_09.pdf).
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## Terrain

.pull-left[
![:width 80%](data:image/png;base64,#images/photo-1469135762633-c299d101636a.jpg)
![:width 80%](data:image/png;base64,#images/photo-1483699862560-f524a8c11c6c.jpg)
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![:width 88%](data:image/png;base64,#images/photo-1486393658811-7168187b08b7.jpg)
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.caption[
Photos by Martin Sattler, Matt Nelson, Pierpaolo Lanfrancotti on Unsplash
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## 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[
![:width 74%](data:image/png;base64,#images/photo-1502866088342-cc132593eea7.jpg)
![:width 80%](data:image/png;base64,#images/2017-01-15-measuring-08_25_34-.png)
.caption[
sources: Photo by Kasuma F. Gruber on Unsplash; https://digital-geography.com]
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## DEM (Digital Elevation Model)

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* 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[
![:width 50%](data:image/png;base64,#../08/images/spotview.gif)![:width 50%](data:image/png;base64,#../08/images/GUID-6D12B139-0878-494D-ABC4-01C0250FB757-web.png)
.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...

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## DEM Sources

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* 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[
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![:width 80%](data:image/png;base64,#images/Digital_elevation_model_Bachu_China_pillars.jpg)
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.caption[Digital elevation model Bachu, China. Source: https://www.esa.int]
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## 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?)

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![:width 80%](data:image/png;base64,#images/reproject_raster01.png)
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## Reproject Raster

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* 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[
![](data:image/png;base64,#images/1920px-Utm-zones.jpg)
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## 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

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![:width 48%](data:image/png;base64,#images/reproject_raster02.png) ![:width 48%](data:image/png;base64,#images/reproject_raster03.png)
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## Slope

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* 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[
![](data:image/png;base64,#images/GUID-64B478F2-045A-4DDD-9E0E-5757B9098B1B-web.gif) ![](data:image/png;base64,#images/GUID-C5E959E6-B10A-4F96-92EE-DD1E4EF0E4CE-web.png)
.caption[Source: https://desktop.arcgis.com]
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## 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

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![:width 48%](data:image/png;base64,#images/slope01.png) ![:width 48%](data:image/png;base64,#images/slope02.png)
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## 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

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![:width 48%](data:image/png;base64,#images/slope03.png) ![:width 48%](data:image/png;base64,#images/slope04.png)
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## Aspect

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* 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.*
]

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![](data:image/png;base64,#images/GUID-A3590481-6E96-4BD8-BF50-25093FD55F00-web.gif) ![:width 70%](data:image/png;base64,#images/GUID-8A2021F8-6D40-4EDA-B677-4C376C6DC246-web.png)

![:width 70%](data:image/png;base64,#images/GUID-31D00098-3346-46B2-A7E6-DAB0E0EBDF10-web.png)

.caption[Source: https://desktop.arcgis.com]
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## 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

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![:width 48%](data:image/png;base64,#images/aspect01.png) ![:width 48%](data:image/png;base64,#images/aspect02.png)
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## 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?

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![:width 48%](data:image/png;base64,#images/aspect03.png) ![:width 48%](data:image/png;base64,#images/aspect04.png)
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## TPI
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* 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**
]

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![](data:image/png;base64,#images/The-Topographic-Position-Index-Definition-after-3.png)
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## Calculating TPI in QGIS

* Go to 'Raster > Analysis > Topographical Position (TPI)'
* you actually can't change anything here
* Click Run

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![:width 48%](data:image/png;base64,#images/tpi01.png) ![:width 48%](data:image/png;base64,#images/tpi02.png)
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## 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

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![:width 48%](data:image/png;base64,#images/tpi03.png) ![:width 48%](data:image/png;base64,#images/tpi04.png)
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## Landforms

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* significance of the TPI depends strongly on size of the neighborhood
* comining TPI from different neighborhood sizes reveals more 'natural' land form structures
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.pull-right[
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![](data:image/png;base64,#images/tpi_ex01.png)
![](data:image/png;base64,#images/tpi_ex02.png)

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![:width 50%](data:image/png;base64,#images/tpi_ex03.png)
.caption[Source: http://www.jennessent.com]

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## Calculating Landforms in QGIS

* Not available from the menue
* Open the Toolbox ![](data:image/png;base64,#images/toolbox_icon.png)
* 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...

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![:width 48%](data:image/png;base64,#images/landforms01.png) ![:width 48%](data:image/png;base64,#images/landforms02.png)
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## Calculating Landforms Result
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* 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
]

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![:width 43%](data:image/png;base64,#images/landforms03.png) ![:width 43%](data:image/png;base64,#images/landforms04.png)
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## '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

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![:width 43%](data:image/png;base64,#images/basic_terrain01.png) ![:width 43%](data:image/png;base64,#images/basic_terrain02.png)
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# What We've Covered
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+ Basics of Terrain Analysis
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+ Slope
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+ Aspect
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+ TPI
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+ Landforms

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## More Terrain Analysis using SAGA

.pull-left[
Olaya, V. (2004): A Gentle Introduction to SAGA GIS.
http://downloads.sourceforge.net/saga-gis/SagaManual.pdf
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![:width 90%](data:image/png;base64,#images/1402459962.png)
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## Homework

* Get the SRTM data from Ireland
* calculate the TPI
* send me a screen shot

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# Any questions?

![:width 20%](data:image/png;base64,#images/1*mYaHTbioZgltE6g7-btVGA.jpg)

.caption[Source: https://www.instagram.com/sadtopographies]

.footnote[
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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>
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