GIS Training & Research Center, 921 S. 8th Ave, Stop 8104, Idaho State University, Pocatello,
Keith T. Weber
GIS Training & Research Center, 921 S. 8th Ave, Stop 8104, Idaho State University, Pocatello,
Idaho 83209-8104 email@example.com (corresponding author)
Department of Geosciences, Idaho State University, Pocatello, Idaho 83209
Daniel P. Ames
Department of Geosciences, Idaho State University, Pocatello, Idaho 83209
Robin E. Patillo2
Department of Nursing, Idaho State University, Pocatello, Idaho 83209
GEOSPATIAL ANALYSIS OF TREE ROOT DAMAGE TO SIDEWALKS IN
Trees are often considered the primary cause of sidewalk damage in urban settings. This study
compared existing sidewalk damage areas to the location of trees in the cities of Pocatello and
Chubbuck, Idaho. Locations of sidewalks and sidewalk cracks were collected in summer of 2007
using a handheld GPS receiver. Quickbird satellite imagery was acquired for the study area in
April, 2008. Using HotSpot analysis, the areas having the highest sidewalk crack density were
current contact, Cottey College, 1000 W. Austin, Nevada MO 64772 (firstname.lastname@example.org)
current contact, College of Nursing, University of Iowa, Iowa City, Iowa 52242 (robin-
identified and a five-block area was subset from both old (average home construction age >20
years) and new neighborhoods (average home construction age < 10 years). Tree canopies were
manually digitized and the drip line perimeter was used to determine the percent of sidewalk
cracks intersecting these polygon features. The results revealed that only 17% of cracks in old
neighborhoods were directly associated with existing tree roots whereas, in new neighborhoods
the percent incidence dropped to 3.5%. Our findings indicate that trees were not the primary
cause of sidewalk damage in the study area and have potential implications for management of
municipalities beyond the study area.
Urban forests can be defined as ecosystems that emerge due to the presence of trees and other
vegetation in association with human development (Nowak et al. 2001). They are an important
asset in the urban areas where 80% of the U.S. population lives (Dwyer et al. 1992, U.S. Census
Bureau 2000). Urban dwellers may plant trees for a number of reasons. Some plant trees because
they are motivated by personal and environmental value systems. Others are motivated by more
practical reasons, such as noise reduction, shading to reduce watering costs, and increased
property values (Westphal 1993). Sommer et al. (1994) demonstrated that people plant trees
because trees were perceived to improve neighborhood interaction and empower residents to
improve their own surroundings. A more recent study by Lohr and Pearson-Mims (2002) showed
that urban residents held positive attitudes toward trees. These attitudes were even more positive
if the homeowners took part in gardening and tree planting.
Despite the advantages of trees, Lohr et al. (2004) identified a number of problems including:
allergies, obstruction of street signs, damage to power lines, increased concealment for criminal
behavior, sap damage to automobile finishes, perceptions that trees are unsightly when not
maintained, the perception that trees cost cities too much money, and that tree roots are the
principal cause of cracked sidewalks. This study focused upon quantitatively assessing the latter
Different species of trees have different types and extents of root systems. However, the majority
of trees have root systems which extend down and outwards in balance with the top growth of
the tree (Kohut 2007). As a rule of thumb, roots extend just a little further than the tree canopy
(i.e., drip line) (Kohut 2007).
Wagar and Barker (1983) found that tree roots can cause major damage to sidewalks and curbs
each year and that repair costs represent a large expense in any city’s budget. Hamilton et al.
(1975) found that annual repair costs due to root-damaged sidewalks were $27,000 each within
22 northern California cities. Sidewalk damage was especially serious as cities were increasingly
liable when a citizen was injured due to a damaged sidewalk (Samuel and Radkov 1977, Edgar
1962). Over two decades later, McPherson (2000) reported approximately $70.7 million was
spent annually by 18 California cities on “tree-root related costs” (sidewalk repair [$23 million],
curb and gutter repair [$11.8 million], trip and fall liability payments and legal costs [$10.1
million]). Their study was based on a mailed questionnaire.
On the other hand, Sandfort and Runck (1986) and Sandfort (1997) suggested that other factors,
such as soil characteristics, may be more important relative to sidewalk failure. In addition,
Sydnor et al. (2000) found that only one of their three study sites exhibited sidewalk damage
attributable to tree roots. They concluded that trees appear to play only a minor role in sidewalk
service life. Further results suggested that sidewalks older than twenty years failed at a higher
rate regardless of any other factors. Sidewalks that were less than twenty years old and built on
fine silt or fine loam soils appeared more stable and less prone to failure compared to those
constructed on coarse or mixed soil complexes. Newly built sidewalks that were less than five
years old were not affected by trees in any type of soil examined. Sydnor et al. (2000) concluded
that trees may have less of an impact than previous studies suggest. Sydnor et al. (2000)
acknowledged that trees can displace sidewalks, but may not be the principle cause.
D’Amato et al. (2002) related that sidewalk engineers in Cincinnati, Ohio considered sidewalks
to last a period of 20 to 25 years and not indefinitely. Furthermore, it was pointed out that
sidewalk construction methods have changed over the years. In the past, engineers were required
to build sidewalks that were 13 cm thick using a gravel base which was inspected during and
after installation. Currently sidewalks are constructed approximately 10 cm thick. Additionally,
and as a common cost-saving measure, sidewalks are inspected only after installation and are not
required to have a gravel base (D’Amato et al. 2002). This suggests the need for further studies
exploring the cause of sidewalk cracks relative to the presence of tree roots.
This study was specifically designed to address the uncertainties described above and determine
the role of trees/tree roots on sidewalk failures by quantifying the geospatial relationship
between the location of known sidewalk cracks and trees/tree roots.
This study was conducted within the cities of Pocatello (total population 52,443 [U.S. Census
Bureau, 2008]) and Chubbuck, Idaho (total population 9,700 [U.S. Census Bureau, 2000])
(Figure 1). In these cities, sidewalks are found along nearly all city roads suggesting that a large
number of people could use them on a daily basis. However, if sidewalk conditions are
hazardous (having cracks and/or obstacles such as trees, poles, and other objects) then people,
especially those with disabilities, will face problems and potentially avoid using sidewalks.
Figure 1. Study area of Pocatello and Chubbuck Idaho. An example of a) new neighborhoods,
and b) old neighborhoods.
A census of all sidewalk cracks within the study area was completed during the summer of 2007
using a Trimble GeoXH GPS receiver (+/- 0.20 m @ 95 % CI). A total of 479 km (297 mi) of
street network sidewalks were documented along with 5,804 sidewalk hazards. The total length
(479 km) represents all collected sidewalks (vector line data) determined using the “Calculate
Geometry” tool in ArcMap. Quickbird (0.6 m) high spatial resolution panchromatic imagery was
acquired in spring 2008. This imagery consisted of two scenes that cover the study area (Figure
Figure 2. An example of Quickbird (0.6m) panchromatic satellite imagery: a) Chubbuck and
north Pocatello, Idaho, and b) south Pocatello, Idaho.
To assess the effect of tree roots as causal agents of sidewalk cracks, the identification of areas of
high sidewalk crack concentration was needed. To identify such areas, HotSpot analysis using
sidewalk crack point data was used to indicate where cracks were spatially clustered. Two
HotSpot areas were extracted for further investigation, one within areas of older neighborhoods
(average age of home construction > 20 years [Byington, pers. comm. 2007]) and a second
within areas of new construction (< 10 years). Old neighborhoods were used to better ensure the
inclusion of mature trees with relatively extensive root systems. Results from old neighborhoods
were compared to new neighborhoods.
Based on HotSpot analysis, a 5 block by 5 block area was selected from the old neighborhoods.
A rectangular polygon covering the 5 x 5 block area was digitized and used to extract a sub-
image from the Quickbird imagery. A sub-image of the same size within new neighborhood
areas was created following a similar procedure. Each neighborhood polygon covered
approximately 317 km2 (122 mi2) and included houses, backyards, roads, sidewalks, etc. (figure
3). The color scheme (green, yellow, orange, and red) represent increasing intensity of sidewalk
crack clusters based on hotspot analysis. The hotspot analysis figure shows, that few areas of
very high concentration (red spots) existed. These happen to overlap less than the more
numerous orange spots.
Figure 3. An example of sub-image extraction along with sidewalk cracks and HotSpot results'
for a) old and b) new neighborhoods. Concentration of cracks increases from green, yellow to
Using Quickbird imagery, all tree canopies within old and new neighborhood areas were
digitized (Figure 4). The ArcGIS intersect tool was used to identify cracks inside and outside the
drip line polygons. Cracks within the drip line were assumed to be associated with root impact.
Figure 4. Digitized tree canopies in: a) old and b) new neighborhoods are shown in cyan
Sidewalks in old neighborhood had 4.5 times more canopy cover (12.4 km2 [4.8 mi2]) than
sidewalks in new neighborhoods (2.7 km2 [1.0 mi2]). In addition, old neighborhoods had 2.3
times as many cracks (n = 262) as new neighborhoods (n = 112) (Table 1). While the results of
these analyses showed an increased number of hazards associated with tree roots within old
neighborhoods relative to that found in new neighborhoods (Table 1), the proportion of sidewalk
hazards attributable to tree roots was low in all cases.
Total area of neighborhood polygon (km2) 317 317
Tree canopy cover (km2) 12.4 2.7
Sidewalk cracks (total) 262 112
Sidewalk hazards intersecting tree canopies 44 (17%) 4 (3.5%)
Table 1. Sidewalk hazards and results of tree drip line intersection analyses
While only 6% of all sidewalk cracks located in the study area (n = 5,804) were included in the
old and new neighborhood sub-set areas, these areas represented the highest concentration of
sidewalk cracks and are believed to be representative of the larger study area. However, this
assumption was not tested and results may vary if the study were repeated on a larger scale.
DISCUSSION AND CONCLUSIONS
Results of this study indicate that tree roots were not the primary cause of sidewalk failures in
the study area. Using the most critical estimates, less than 4% of sidewalk cracks were located
within tree drip lines in new neighborhoods. Similarly, in old neighborhoods, only 17% of cracks
were located within tree drip lines. Our results suggest that other factors, such as those discussed
by Sydnor et al. (2000), contributed to the sidewalk cracks observed in the Pocatello and
Chubbuck study area. Some of these factors may include soil type, sidewalk construction
techniques, freeze-thaw patterns, and the effects of time and use. Our work confirms the Sydnor
et al. study (2000). Additionally, our findings are of particular interest as these results
demonstrate consistency across relatively different geographies (Cincinnati, Ohio and
Pocatello/Chubbuck Idaho) exhibiting different soil types, climates, and eco-regions, even
though very different analysis techniques were used. Further analysis is still needed, however to
determine the primary cause of sidewalk cracks but the fusion of GPS and remote sensing data,
coupled with GIS analysis may help answer this question as well as other issues related to urban
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and costs of the urban forest. Journal of Arboriculture, 18(5):227-234.
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Interaction by Design: Bringing people and plants together for health and well-being: An
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ABOUT THE AUTHORS
Mansoor Raza is a graduate of Idaho State University where he earned his Master’s in
Geographic Information Science from the Department of Geosciences. Mansoor is
currently working as a GIS Technician II in Canada. His research interests include municipal
applications of remote sensing, GIS for urban and rangeland management, GIS
Application/Analysis for asset management, QA/QC of map documents, Web GIS mapping,
Object Oriented GIS, and feature extraction.
Keith T Weber is the GIS Director at Idaho State University where he leads the GIS Training and
Research Center. Weber, a certified GIS professional (GISP), has published 30 papers in peer-
reviewed professional journals with focus on remote sensing and geospatial analysis of semiarid
Keith T. Weber, GISP, GIS Training & Research Center, 921 S. 8th Ave., Stop 8104,
Pocatello, Idaho 83209-8104. e-mail: email@example.com
Dr. Sylvio Mannel is currently an Assistant Professor at Cottey College, where he is in the
process of setting up a new four-year Environmental Studies program. He earned his Doctorate
degree at South Dakota School of Mines and Technology. His research interests include
biogeographic and interdisciplinary applications of geotechnology, such as GIS, remote sensing
and spatial analysis.
Dr. Daniel P. Ames received his Ph.D. in Civil & Environmental Engineering in 2002 from Utah
State University. His research interests include watershed modeling, decision support systems,
Bayesian decision networks, time series analysis, and GIS tool development.
Robin E. Pattillo PhD, RN, CNL is currently employed as a Clinical Associate Professor in the
College of Nursing at the University of Iowa in Iowa City, Iowa. She received her PhD in
Exercise Physiology from Auburn University in 2000. Her research interests include technology
in nursing education, health promotion, the impact of the environment on health, and the
application of GIS to the evaluation of health related resources.
This study was made possible by a grant from the Bannock Transportation and Planning
Organization and the National Aeronautics and Space Administration Goddard Space Flight
Center (NNG06GD82G). Idaho State University would like to acknowledge the Idaho
Delegation for their assistance in obtaining this grant.