When tree roots and impervious surfaces meet each other, disaster is usually the result. First, the concrete house foundation, driveway, sidewalk, or sewer line gives way to the tree roots--disaster for the surface and its owner, and then in retaliation, the tree may well be cut. The problem of course is that the long-range pattern of tree root growth was not anticipated by the original developer of the parcel of land. Most people think only of how the trees and shrubs look, and how they will enhance property sales, at the time of construction. The cumulative effect of initial lack of consideration of this problem leads to disaster years later: disaster to homes, to sidewalks and sewer lines, and, to tax base that gets gobbled up in law suits. All of this could have been avoided by planning for trees that considered the problem of conflict between roots and surfaces and adopted a strategy for minimizing or resolving this conflict. (Ann Arbor's Natural Features Ordinance (Chapter 57 in Ann Arbor City Code) does not apply to single family lots--this sort of information would encourage homeowners, for their own interest, to plant reasonable trees (especially when coupled with the City list of invasive species)). Indeed, as cities age and get rebuilt, it is as important then, as it is for the emerging pristine development, to consider this problem.

The research question is to develop a strategy that minimizes or resolves the conflict between these two sets of systems (one natural, one human). One strategy that would address this problem is to map existing neighborhoods based on the geometry of the infrastructure of existing housing stock (footprint/rooftop and driveway), sidewalks, street curb, water and sewer lines, and (perhaps) overhead lines. Distance contours at a 1 foot (or other) interval from each of the chosen items would be the base from which a critical root zone map would be created. Follow-up with the Homeowners Association and posting of information on their website would encourage individual parcel owners to consider the merits of planting not only non-invasive species (with the list from the City) but also plantings with correct root ball size (information from local nursery personnel or land development coordinator) to avoid future difficulties with impervious surfaces.

The neighborhood for proposed pilot study is the Bromley neighborhood in Northeast Ann Arbor: 231 homes bounded by the south on Huron Parkway, the west by Nixon Road, the north by Bluett, and the east by Prairie.

HYPOTHETICAL SAMPLE MAPS

Hypothetical analysis can often suggest how actual analysis will appear.  There may well be merit in spending a few minutes in creating hypothetical maps prior to investing large amounts of time in creating actual maps.  In this section, hypothetical maps are created.  In the following section, Lloyd R. Phillips made actual maps.

Bromley subdivision from the City of Ann Arbor parcel map

House footprints (hypothetical here--to be digitized from orthorectified aerial photos).

Distance from houses: in the yellow 40-45 foot zone one could plant a tree with a mature root system of less than 45 feet without having that system reach the nearest house.

Other human systems should be digitized and overlain on the house footprint root ball contour pattern; here is a sample with hypothetical infrastructure/sewer line.

In the detailed view above, one sees the gradation in away-from-the-house contours. The size of each is 5 feet. Homeowners who are aware of these distances and unseen difficulties would likely wish to choose planting patterns to avoid future harm/expense to their impervious surfaces. Overlaying various layers creates a pattern that is more intricate than that shown here and it is that pattern, that tells where it is safe to plant trees with respect to root distance from impervious surfaces. The maps offer a critical root zone geographic-signature of the neighborhood. With such a long-range planning effort, the homeowner wins, the City wins, and the trees win.

ACTUAL MAPS OF BROMLEY

During the Fall of 2000, co-author Phillips worked to implement the strategy above for one neighborhood well-known to co-author Arlinghaus.  While the purpose of the project was to offer a map of regions, as a pilot project, in which trees of varying sized rootballs might be successfully planted (without interfering with house foundations, sidewalks, or driveways), there was also the added benefit of producing a map of previously undigitized impervious surface and using the GIS to calculate percentage of impervious surface coverage.  Impervious surface location, and percentage by watershed, is important in understanding regional stream health.  Information of this sort appears to becoming of increasing significance in coming to local policy decisions.  Note the general impervious surface map, from SEMCOG (South East Michigan Council of Governments) for all of Washtenaw County (county containing Ann Arbor).  The white areas are areas for which there was no data.

Rectified-orthophotos (raster data) were obtained from Merle Johnson, Information Services Division, City of Ann Arbor. In 1992,  Woolpert of Ohio began the functions necessary to create orthophotos.  The work was completed in 1993-1994.  In 1997, an update was completed for selected areas and a complete fly-over of the city was undertaken.  The image created is a 1' resolution digital orthophoto measuring 30" x 30" at a scale of 1" = 125'.  Digitizing of the photo was done using ArcView 3.2 (ESRI) at a scale ranging between 1:200 and 1:300.  For more detail on the photos, see the accompanying Appendix.

City of Ann Arbor parcel map superimposed on aerial photo.
 

Taking a closer look reveals the sort of detail available.
 

Digitizing (by Lloyd R. Phillips) of rooftops, sidewalks, and driveways at various contour intervals.

Two-foot contour interval: image does not show entire neighborhood.
 

Five-foot contour interval:  image does not show entire neighborhood.
 

Ten-foot contour interval:  image does not show entire neighborhood.
 

Twenty-foot contour interval, rooftops only:  image does not show entire neighborhood.
 

All taken together, 2, 5, and 10 foot contour intervals:  image does not show entire neighborhood.
 

Thus, one could choose a planting scheme, based on rootball size, designed not to interfere with existing concrete.  This choice would be made simply on distance.  Mr. Phillips also discovered that there are business firms that recognize this difficulty and can install an underground shield so that trees larger than distance would suggest prudent, can be planted closer to concrete.  In either case, once again, trees win, property owner wins, and City wins.
 

Appendix

In this region,  there were no tall buildings or other structures to cause a problem with vertical displacement.  One problem  to be resolved, however, was the distortion of shapes due to the sun angle and the object-distance from nadir.  The features that exhibit this phenomenon were sidewalks.  The widths of the sidewalks often vary in shape; whereas, they were digitized as they were perceived.

Lack of photons (or energy) reaching the sensors in the aircraft due to obstruction, was a common problem, caused primarily by building or tree shadow.  Thus, the image's spectral resolution becomes diminished.  Exact shape and position of individual house rooftops were therefore difficult to determine.  Additionally, the same shadows would, on occasion, obstruct the outline of the driveways and associated impervious structures.

These considerations were factored into the digitizing process.  Further detail of the entire survey is available from the City of Ann Arbor.  A few details of note include:

• The geographic extent of the digital orthophotos is 50+ square miles from M-14 on North to Morgan Road on the South.  The range extends from Wagner Road on the West to Gale Road on the East.
• Standard digital orthophotos are cast on the Michigan State Plane - South Zone projection on the North American Datum of 1983 (NAD83) with coordinates in feet.
• The ordering of the data is by lines (rows) and samples (columns) with each line containing a series of  pixels ordered from west to east. The order of the lines is from north to south. When displayed on a computer, the image projection grid north is at the top.
• The four primary datum (NAD83) corners are imprinted into the image as four solid white crosses with an image value of 255 and the four secondary datum corners as four dashed white crosses with similar intensity values.