A GIS Examination of the Chacoan Great North Road

Abstract

This study uses geographic information systems (GIS) to examine the prehistoric road corridor that stretches north from Chaco Canyon to the San Juan River in northwestern New Mexico. Termed the “Great North Road,” this roadway has been extensively examined on the ground and with overhead photography and sensors. We apply GIS abilities to quantify descriptive measures of location, direction, and elevation and, in combination with previous work, compile the information to assess basic engineering choices made by the Chacoans in creating the road.

El presente estudio usa Sistemas de Información Geográficos (SIG) para examinar el corredor prehistórico norteño que va desde Chaco al río de San Juan en el noroeste de Nuevo México. Llamado el “Gran Camino del Norte” esta vía ha sido extensamente estudiada por vía terrestre, fotografía aérea y distintos sensores. Nosotros aplicamos múltiples capacidades del SIG para cuantificar medidas descriptivas de localización, dirección y elevación, además de combinarlo con trabajos previos, compilamos esta información para evaluar las decisiones ingenieriles básicas tomadas por los habitantes de Chaco para construir el camino.

KEYWORDS:

• Chaco
• Prehistoric Roads
• Geographic Information Systems
• Great North Road
• Chacoan Wide Roads
• Least-Cost Paths
• Hiking Functions
• GIS

Notes

1 To assess the efficiency of alternative routes two travel cost functions were used, the Tobler function (Tobler 1993) and the Minetti function (Minetti et al. 2002). Both of these create measures according to the slope of the ground crossed. For the calculations presented in this study the slope figures use are for 30-meter sections of terrain.

The Tobler expression:

T = D/(6 exp(−3.5 × abs(S + 0.05)))

where:

T = time required to traverse distance D

D = distance

S = slope of the ground over distance D

The Minetti function:

E = 1337.8 S⁶ + 278.19 S⁵ − 517.39 S⁴ − 78.199 S³ + 93.419 S² + 19.825 S + 1.64

where:

E = energy expended to cross the section of ground with slope S

S = the slope of the section of ground

A measure of velocity of walking is the result of the Tobler function, while a measure of energy expended while walking is the result of the Minetti function. Each of these functions has been used in archaeological examinations of alternative travel paths. Of note, both Kantner (1997) and Heberling (2010) use Tobler cost path calculations to assess Chacoan wide road locations in the southern Chacoan region. Work using Minetti calculations include that of Herzog (2013).

Analysis using these functions must assume the direction of travel measured because both the Tobler and Minetti functions are isotropic, that is, their results are not symmetrical for upward and downward slopes. In both functions the most efficient slope for travel is for a slight decline in elevation. North Road analysis in this paper assumes a direction of travel from north to south; this reflects the likelihood that the prehistoric flow of most resources would have been to Chaco Canyon from the north.

Both of the cost functions are used in this paper contrast the route of the Great North Road with hypothetical alternative routes. While no simple mathematical function can be expected to precisely measure either the time required or the energy expended for foot traffic over varied terrain, a ratio of these measures for one route versus another should provide information valuable in comparing the routes' efficiencies. The cost functions for this study are used in this light – not as precise time and energy figures in themselves, but as ratios, one route to another.

2 These database entries incorporate locations from Phase I, other varied research observations, and field GPS readings by the authors. The route length in these three systems are virtually identical, although slight variation can be noted due to recent photography in Google Earth Pro recording differences in the Kutz stream bed from earlier topographical map surveys. Regarding distances, in addition to the linear map distances, GIS software also provides measures of the 'terrain distance' – distance including ups and downs in the terrain. For the elevation changes involved for the North Road, however, terrain distances would be minor are not included. (Elevation change distance would, for example, only amount to about 100 meters over the entire North Road).

3 Elevation profile figures are not to scale as vertical elevation is exaggerated. Elevation profile data from alternative GIS measurements are virtually identical. Base elevations can differ slightly according the mapping source. Elevation in Google Earth presentations for this area is from satellite radar measures, SRTM, with 16 meters vertical accuracy of 90% confidence; the relatively lowest accuracy in this measure occurs for high slope areas and areas of heavy vegetation. TOPO elevation base is from Digital Elevation Models (DEMs) produced by the United States Geological Survey (USGS). ArcGIS accesses elevations from a variety of sources.

4 Current New Mexico State Road 57 is labeled “Road 56” on older maps; the road is the same but the old number “56' was officially changed to avoid confusion with US Route 56.

5 Some divergence between some measures with the Phase 1 may be noted. It is possible that typos may have been present in printing the Phase 1 pages 6–27 and Table 6-3, and the location of the roadways in the description may not have been precisely specified. (The description given of the southern map boundary, for example, should likely be Section 19, T. 22 N., R. 10 West not Section 19, T. 22 N., R. 10 East).

6 Likely the hiking function understates the difficulty of the rise out of Kutz Canyon at this point. In fact, making the climb out of Kutz Canyon at this point would have been (as it is today) impossible without considerable construction effort and/or use of climbing equipment. In this regard, the detailed Phase I study of this location describes the North Road drop into Kutz Canyon wash as: “…a dramatic descent. There is no evidence on the ground to suggest how this descent was accomplished.” (Phase 1, p. C-54)