Getting a Good Spatial Location
By Casey McLaughlin
How can we get an accurate spatial location for mapping property sampling? Is an address enough? Is collecting a latitude/longitude location enough? EPA samples properties for a variety of reasons but for the sake of this post, let’s assume we are sampling several properties adjacent to a recent chemical fire. Samplers get property access by knocking on doors and having owners sign an access agreement. After completing their work, a sampler will have a field sheet, an address, and often times a latitude and longitude (from GPS). Field work done!
The job is not quite finished, however, until we turn that information into actionable intelligence – information we can use! For me, the best way to use the field information is to first see it displayed in a map. Others may see this data in other formats, but a map is a great tool for understanding the situation. Does a map show us that the location data collected in the field is good enough? Let’s look at this example to find out:
Figure 1 highlights three pieces of information that we like having: Parcel Lines, GPS locations, and the final points. The parcel lines can be a great source of information and look great for making maps, but are not always available for all projects. I prefer starting a project with parcels and narrowing the relevant area down, but that is not usually possible. What I often get from field staff are either addresses or latitude/longitude coordinates. In my opinion, both have their issues as shown by the blue dots on the map. Look at blue points B and blue C. The GPS location could have been taken from the end of the driveway where there are fewer trees, but on the map I might find it difficult knowing which polygon is B and which is C. The white B and C points clearly illustrate their relationship to each residential dwelling.
Take note of how zooming the map out (figure 2) will quickly cause overlapping points. Also note that only three samples were taken (perhaps not everyone was home), therefore it would be easy to overlook the other properties in the cul-de-sac that may need sampling. Matching points with an authoritative properties dataset can easily show us which properties may still need sampling.
There are a number of mechanisms for determining spatial locations, including GPS, mobile phones, surveying, and addresses. Each can be used, but choosing the right method should be determined by considering both the immediate field needs and long-term project needs such as creating general site maps, point maps, area maps, or public information maps.
Casey McLaughlin is a first generation Geospatial Enthusiast who has worked with EPA since 2003 as a contractor and now is the Regional GIS Lead. He currently holds the rank of #1 GISer in the EPA Region 7′s Environmental Services Division.
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