Electromagnetic Fields Modeling Using GIS
Three dimension terrain information is essential for the determination of E-field and B-field in
the region. The strengths of E-field and B-field are a function of r which is the distance between
the power line and the point of interest. With the availability of the digital terrain model, the (x,
y, z) coordinates can be used to compute the strengths of E-field and B-field in the region. The
locations and the heights of the towers can be obtained from the electric company. By using the
equivalent charges method as described in the last section, the strengths of both E-field and B-field
can be determined. Hence, for the study region, there are three TIN surface models. Other
than the original terrain model, there are also one B-field surface model and one E-field surface
model. The strengths of these two fields are processed through a customized program. The
results are imported into a GIS to generate TIN coverages.
Since GIS is a 2D system, it will be difficult to perform analysis on surface models such as TIN
models. One solution is to convert the TIN surface models to lattice surface models. Then, all the
analysis would be performed in raster mode. Based on the magnitude values associated with each
pixel in the models, field strengths are classified into different classes according to table 1. The
shaded records on the table are the field strengths exceeding the safety level and defined to be
‘exposed’. Other than the surface coverages for the terrain, the B-field and E-field, there are also
land parcels, buildings, paths and power lines coverages in the system. These coverages are also
converted to raster form before performing overlay analysis.
Discussions
Figures 2 and 3 from the field guidelines statec llustrate the exposed area in the region. Exposed area means that area is suffered
and the magnitude exceeds the IRPA/INIRC standard. The IRPA/INIRC
that the threshold electric field is 5 kV/m and the magnetic field is 0.1 mT. It is
clear that both fields diminish rapidly with increase in distance. With 23m and 11m away from
the power line, the E-field and B-field would become insignificant respectively. Hence, only the
areas which are very closed to the power lines would experience the fields.
The results (exposed area) as illustrated in Figure 2 and 3 are overlaid with the building coverage
to identifj buildings which are inside the exposed area. The results are shown in Figures 4.
From the analysis, it is found that only one house is affected by E-field and two houses are
affected by B-Field. Similar analysis is performed by applying the 50-metre corridor criterion in
the system. A 25m buffer zone is generated around the power line and houses inside the zone are
identified. It is found that the results are the same as the previous analysis.
From the above results, one can conclude that GIS can be used for route planning of overhead
transmission lines with the use of a proper ELF-EMF emission model. The strengths of both B-field
and E-field in the vicinity of the proposed power lines can be determined with the
availability of a digital terrain model and the proposed locations of the towers. The B-field and
E-field surface models generated can be used to identifj the “exposed area” in the region. By
converting the TIN surface models to lattice surface models, overlay analysis can be performed
with coverages such as land use coverage, building coverage. Properties and buildings which are
inside the “exposed area” can easily be identified. Owners of the properties can be notified to
claim for compensation. Planners and engineers can also use the information to assess the
number of possible claims and estimate cost for compensation. Alternative routes can easily be
introduced into the system once the coverages are available. An optimum route can be found by
altering the locations of the towers to stay away from in residential zones.
However, as both fields are very sensitive to distance between the point of interest and the
location of the power line. As for the case of planning, digital terrain model(DTM) is usually
acquired from small to medium scale topographic maps. The DTM generated from this type of
data will definitely not be very accurate. Nevertheless, as long as the planners and the engineers
are aware of the fact, the information provided from this system can still be very usefid. Of
course, more reliable results can be obtained if an accurate DTM is provided.
References
- Ahlbom A., et al., (1987) Biological effects of power line fields. In: New York State Power
Lines Project, Scientific Advisory Panel Final Report. New York: New York State. pp. 67-87.
- Carpenter D. and Ayrapetyan S., (1994) Biological Eflects of Electric and Magnetic Fields:
Source and Mechanisms, Academic Press.
- Duchene A., Lakey J. and Repacholi M., (1991) IRPA Guidelines on Protection Against Non-Ionizing
Radiation, Pergamon Press, pp. 83-100.
- Goodland R., (1973) Power Lines and the Environment, The Cary Arboretum of the New York,
New York.
- Smith C. and Best S., (1989) Electromagnetic Man : Health and Hazard in the Electrical
Environment, J. M.Dent & Sons Ltd., London.
- Wertheimer N. and Leeper E., (1979) Electrical wiring configurations and childhood cancer. Am.
J. Epidemiol. Vol. 109, No. 3, pp. 273-284.
- Ueno S., (1996) Biological Effects of Magnetic and Electromagnetic Fields, Plenum Press, New
York.
