Fil: Magnin, Lucía. División Arqueología. Facultad de Ciencias Naturales y Museo. Universidad Nacional de La Plata; ArgentinaFil: Gobbo, Juan Diego. División Arqueología. Facultad de Ciencias Naturales y Museo. Universidad Nacional de La Plata; ArgentinaFil: Gómez, Juan Carlos. Instituto de Recursos Minerales (INREMI). Facultad de Ciencias Naturales y Museo. Universidad Nacional de La Plata; ArgentinaFil: Ceraso, Antonio. División Arqueología. Facultad de Ciencias Naturales y Museo. Universidad Nacional de La Plata; Argentin

Ceraso, Antonio

Gobbo, Juan Diego

Gómez, Juan Carlos

Magnin, Lucía

Naturalis

GIS Model of Topographic  Accessibility 
to South America
Lucía Magnin1, Diego Gobbo2, Juan Carlos Gómez3, and Antonio Ceraso4
➤Keywords: South America, topographic accessibility, first Americans
In this paper we analyze the surface of South American territory to create accessibility models 
(Llobera 2000) related to two different theoretical models of the entrance routes to the South 
American continent, as described in detail in Miotti and Magnin (“South America 18,000 
Years Ago: Topographic Accessibility and Human Spread,” this volume). The accessibility 
models are drawn at a continental scale and seek to incorporate data useful as proxies of the 
environment at the Last Glacial Maximum (LGM) (ca. 20,000–18,000 rcybp) (Miotti 2006).
 The methodology employed here is oriented toward delimiting natural corridors of low 
resistance to pedestrian movement for the South American continent using GIS. It differs 
from the calculation of least-cost paths because it does not show the shortest path to link one 
origin and one destination point. Instead, the corridor is a surface which, owing to its ease 
of accessibility, is potentially usable as a pathway (Cerrillo 2008; Llobera 2000, 2006). The 
objective is to use present topography to model ancient coastlines (Isla and Bujalesky 2008), 
to take into account the extension of glacial masses (Clapperton 1983; Hollin and Schilling 
1981; Rabassa 2008; Stanford et al. 2005) as barriers to passage, and to use topography as a 
surface to calculate access costs (Llobera 2000). 
 The specific objective of this work is to generate two predictive maps, one considering 
rivers as partially permeable barriers, and the second considering rivers and marine coasts as 
movement stimulators. 
 1 CONICET–FCNyM – UNLP, Paseo del Bosque s/n, CP: 1900, La Plata, Buenos Aires, Argentina; e-mail: 
lmagnin@fcnym.unlp.edu.ar
 2 CONICET-FCNyM – UNLP; e-mail: dgobbo@fcnym.unlp.edu.ar
 3 FCNyM – UNLP; INREMI; e-mail: juancarlos@geofoto.com.ar
 4 FCNyM – UNLP; e-mail: antonioceraso@gmail.com
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Data and Methods
The general methodology used here is the Optimal Displacement Model delineated by Fábrega 
(2006). However, we depart from that author s´ perspective because in our model the origin 
points are not archaeological sites, but random points within a grid of polygons that cover 
the study area.
 This analysis does not seek to establish optimal access between sites (for an example of 
this approach see Anderson and Gillam 2000), but to characterize the terrain according to its 
natural accessibility irrespective of the point of origin for the movement, thus generating a 
model for natural mobility. This methodology based on a grid of random points was previously 
applied at a different scale to another study problem by Cerrillo (2008).
Figure 1. The input data, reprocesses applied, and models obtained in the GIS analysis.
 In this contribution we use SIG (ArcMap-9.2 and Grass-6.4) to model the main accessibility 
characteristics that the South American territory could have presented during the LGM. The 
14  Southbound Magnin et al.
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different processes applied can be viewed in the diagram in Figure 1. Owing to the flexibility 
of GIS (Geographic Information Systems) in managing, testing, and revising models, the data 
used here can be revised in the future. The analysis followed through nine steps, as described 
below.
 1. As basic data we used a digital elevation model GEBCO_08 Grid. This is a three-dimen-
sional model of ocean bathymetry and emerged topography with a spatial resolution of 
30 arc-seconds (each pixel measures 945.44 m on each side). These data and all products 
were projected to WGS 1984 UTM Zone 20S.
 2. These basic data were used to model the extent of land area in the LGM. Sea level was set 
at 105 m below present level (Isla and Bujalesky 2008). The resulting grid file has 6109 
columns and 8993 rows. This modeling procedure is a simple device that models shore-
lines broadly; however, it is a simplification of a reconstruction process (Waters 1992, 
Dincauze 2000).
 3. The hydrographic network was modeled by the module ESRI ArcHydro-1.3 via the com-
mand for calculating accumulated flow. The resulting hydrographic network is a raster 
file composed by continuous-flow accumulation values. The lower values do not repre-
sent a significant barrier to the passage, and conversely, higher values constitute a poten-
tial barrier to be crossed by purely pedestrian means.
 4. The extent of glaciers as barriers was incorporated into the model from the scanning, 
georeferencing, and orthorectification of maps published in Rabassa (2008:163) and 
Stanford et al. (2005:338).
 5. From the digital elevation model (2), hydrography map (3) and glacier map (4), two fric-
tion surfaces were generated. In Figure 1, the friction map “5a” expresses extremely low 
accessibility values for glaciers (considered barriers to passage); accessibility values for 
rivers become lower as their cumulative flow values increase from the headwaters to the 
mouths. The friction map “5b” also considers glaciers as barriers to access, but it assigns 
high accessibility values to the network of rivers and a coastline of 50 km around the 
perimeter of the continent, which are considered attractors to movement.
 6. A regular grid of polygons of 300 by 300 km was generated to cover the entire study area, 
and one point was randomly chosen within each polygon (N = 244).
 7. Each of these points in turn was set as the origin to generate a cumulative cost surface 
using the GRASS module “r.walk” (http://grass.fbk.eu/grass70/manuals/html70_user/r.
walk.html). This algorithm is superior to others because it includes Naismith rules for 
calculating cost estimates of specific slope intervals, it makes anisotropic estimates, and 
it resolves the tendency to generate polygonal artifacts in the resulting surfaces using the 
“chess knight’s move” (Whitley and Burns 2007). In Figure 1, the cumulative cost surface 
maps resulting from the use of friction map 5a were named “7a,” and the maps resulting 
from the use of friction map 5b were named “7b”.
 8. Using each accumulated-cost surface map, flow-accumulation surface maps were gener-
ated using the GRASS module “r.watershed,” setting the remaining points of the grid as the 
origin, and establishing the destination point by the accumulated-cost surface map used 
every time. This was achieved using a python script, which automates the process of calcu-
GIS Model of Topographic Accessibility to South America 15
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km
lating the cost maps (r.walk) and flow-accumulation map (r.watershed). 
 9. The final step is summing the accumulated flow surface maps derived from the friction 
maps (“5a” and “5b” in Figure 1).
Figure 2. Dual accessibility models produced by the GIS analysis presented in text and Figure 1.
Results and Conclusions 
Models of general accessibility of America were generated (Figure 2). These do not assume 
points of origin or destination for pedestrian population flow into the continent. They are 
continuous data surfaces with cells containing values that express areas of higher accessibility 
considering the total origin points analyzed.
 This methodology proved to be a useful tool for creating a formal, reproducible description 
of two theoretical models of population movements of the first Americans, which is explored 
in more detail in Miotti and Magnin (this volume). 
We thank César Parcero and Enrique Cerrillo. The project was funded by the grant Pict ANPCYT 1552.
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18  Southbound Magnin et al.


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