Jordan Stutt
Intro to GIS
Assignment 4
3/10/11
1. I have been hired as a consultant by the City of Medford to assess stormwater runoff and
flooding. The region I have been contracted to analyze is the area surrounding the Tufts indoor
athletic facilities. In order to provide an effective analysis, the data sets I use must meet certain
criteria. In order to determine the path of stormwater flow, it is of utmost importance that the
data I use has a high degree of positional accuracy. If storm sewers, roads and other impervious
surfaces are positioned inaccurately, my results will be flawed. Likewise, data sets like land use
must have correctly coded attributes in order to offer helpful information. The completeness of
data will be particularly important when it comes to plotting storm sewers, as missing sections
of data here will be very misleading. In terms of time period, land use and land cover data needs
to be recent in order to ensure that maps reflect up to date surface cover. While the
connectivity of roads does play a role in the flow of storm water, this is the least important
factor within the context of my project.
2. Road Centerline Datasets
a) As the above graphic shows, both the Census2000 TIGER Road Centerline as well as the ESRI
Road Centerline have very low positional accuracy at this intersection (College Avenue and
Frederick Avenue, Medford). The TIGER intersection is over 12 meters from the actual
intersection (according to the aerial imagery), while ESRI plots it only slightly more accurately.
The red line is the Road Centerline data from MassGIS. As demonstrated by the graphic,
MassGIS has a far greater degree of positional accuracy at this intersection.
b)
Unfortunately, the positional inaccuracy of the TIGER and ESRI centerlines shown in the first
image is consistent across my study area. While the MassGIS centerlines appear to be reliable,
the ESRI and TIGER centerlines are often far removed from the actual location of the street, and
in one case above, the TIGER centerline depicts a fictional road.
d) The most up-to-date of these centerline datasets is the one provided by MassGIS. The
metadata for this layer states that the layer is up-to-date to December 2007. The TIGER layer is
based on data courtesy of the 2000 Census, and the ESRI layer, which was published in 2002, is
an enhanced version of the TIGER dataset.
e) Of the three options, the positional accuracy of the MassGIS centerlines layer is the most
suited to my project (as I imagine it would be for any project).
f) As far as the road centerline layers are concerned, I am not particularly interested in the
attribute information available. The positional accuracy and currency of this data are both of
great concern, but in terms of stormwater runoff the attributes of roads are not of great import.
With that said, all streets appear to be correctly labeled in each of these centerline files.
g) These three centerline layers appear to be equal in terms of completeness. I cannot find any
real roads which are plotted by one centerline layer but not another.
3.
Land Use/Cover Datasets
a) The image above shows the Land Use layer file from MassGIS over aerial imagery of the area of
interest. Residential areas are accurately coded and positioned (yellow and purple), as are open
space (green), commercial (pink), recreation (blue) and institutional (tan) areas. On the other
hand, the NLCD Land Cover layer file (as shown below) has very poor positional accuracy. The
different categories represented offer minimal help for me due to their apparent lack of
connection to the land cover that actually exists.
b) The displacement between NLCD land cover layer file and the actual land cover that lies beneath
that layer is consistent throughout my study area. The scale at which this layer file was created
is simply useless for a project requiring details such as mine. Meanwhile, the MassGIS land use
layer file shows almost zero displacement from what is displayed by the aerial imagery.
d) The MassGIS land use layer file was created based on aerial imagery from 2005, while the NLCD
layer file is from 2001. As I am interested in the most current information, the MassGIS layer file is
best for my project in terms of currency.
e) Again, the positional accuracy of the MassGIS file is far superior to the NLCD file at this level. For
a project of much larger scope, the NLCD file could be adequate, but for this level of zoom it is
ineffective.
f) The MassGIS file is better for my project than the NLCD file based on attribute information. The
NLCD file has put a large emphasis on categories based on different types of development, while the
MassGIS file uses a simpler, more direct set of land use categories. When it comes to projecting
stormwater flow, the type of development happening to a tract of land is not very important. That
being said, differences between residential and commercial areas (as expressed in the MassGIS file)
are particularly important for this project either.
g) I would have to say that the MassGIS file is more complete than the NLCD data, due to the
precision of the data and the presence of helpful metadata (no metadata has been created for the
NLCD file).
4) Optional Layers
a) Impervious Surfaces Layer: This layer has a very high degree of positional accuracy. Despite
zooming in so closely in the image below, the positional accuracy remains high. It appears that the
only problem is that the data is simply not precise enough (1 meter pixel size) to be perfect at this
level of zoom, but it is excellent positionally.
b) Of my four optional layers, Impervious Surfaces is the only one with easily quantifiable positional
accuracy. Therefore ,the positional accuracy of the Storm Sewers, Topographic Contour Lines and
Soil Slope layers must be measured in qualitative terms.
Storm Sewers: This layer file actually plots both storm sewer drains as well as manholes. The
locations of storm sewer drains are virtually indetectable from the ArcGIS Aerial Imagery layer, so I
did some on-foot detective work and it appears that the Storm Sewer layer is accurately plotted. In
order to check the layer’s positioning of manholes I compared their location to images from google
maps (which is impressively clear at very high levels of zoom). Manholes appear as glaring circles on
google maps, and appear to be in nearly the exact same location as where they are plotted by the
Storm Sewer layer.
Contour Lines: This layer provides topographic contour lines at 30 foot intervals. As a result, there
are only two lines which fall in my project area (at 30 and 60 feet above sea level). If I were to add
an additional contour line data layer to the map I could make a comparison between the two, but
this would not tell me the positional accuracy of the layer file I used, it would merely show me a
difference.
Soil Slopes: Measuring the positional accuracy of a soil slope map appears to be a fruitless
endeavor, particularly without a more detailed set of topo lines. Even from a qualitative standpoint,
it is hard to make an educated guess at how positionally accurate these soil slopes are.
c) Completeness
Impervious Surfaces: This layer file appears to be nearly complete. Because it is a simple, two
category map, the attribute section is very clearly complete. However, there are sections of roads
and walkways which should be categorized as impervious surfaces that are not categorized as such.
Storm Sewers: This layer file is glaringly incomplete for my area of interest, as shown by the below
image. This is surprising, because it is a very detailed data set, and yet there is no data for half of
Medford. Furthermore, a more ‘complete’ attribute table would distinguish between storm sewer
drains and manholes – this layer’s data does not.
Contour Lines: For the purposes of my project, this layer file is lacking. It would be far more helpful
if this dataset used more precise contour intervals than 30 ft. If it were possible to show 5 ft.
contour intervals for example, I could determine the gradients and predict the direction and speed
of storm water flow. With 30 ft. gradients, I can say that water will most likely flow in a northeast
direction across my area of study – not very helpful.
Soil Slopes: This is yet another layer file lacking completeness. It is certainly helpful to an extent;
being able to quantify the slope of the soil into quantiles is important for my project, and this data
layer does allow me to see where soil is steepest. The two glaring issues are that this data layer does
not allow soil slope to be displayed as graduated quantities, and that a great deal of area is coded as
“Urban Land” which does not have any slope value available. As a result, I can only display three
categories of soil slope percentages in my area of interest: 3-8, 8-15 and 15-25. Ideally there would
be many more graded quantities. Furthermore, the image below shows the large area for which
there is no soil slope data, coded “Urban Land.”
d) Currency
Impervious Surfaces: This layer was produced by MassGIS in 2007 using orthoimagery from 2005.
This information is available on the MassGIS website.
Storm Sewers: Unknown. The metadata for this layer file is blank under the category of “Time
period for which the data is relevant.”
Contour Lines: This layer was produced by MassGIS in June of 2003 according to their website.
Soil Slopes: This layer was produced in May of 2007 by the NRCS (metadata).
e) Attribute Information
Impervious Surfaces: The attribute information supplied with this layer is well suited to my project
needs. Although it would be helpful to see a grade scale of impermeability, this is adequate.
Storm Sewers: As I mentioned earlier, the attribute information for this layer which would be
relevant to my project is lacking. Distinguishing between manholes and storm drains would make
my job significantly easier.
Contour Lines: Again, more precise contour intervals would change this from a relatively useless
layer to a very helpful one for me. The lack of depth of this attribute table greatly hinders its
effectiveness for my project.
Soil Slopes: Being able to set my own classifications for this layer would make it far more useful. As
it is, its use for my project is limited because A) the classes available are not specific enough and B)
there is a great deal of uncoded territory.