Jeremy Robitaille
UEP 232
Assignment 5 – GIS Data Quality Assignment
1. Provide one-paragraph description of the project you are using as a benchmark to assess
the data and what positional accuracy it will require (or what is good enough - think
about how far off the position could be and still work for the project needs)
This GIS Data Quality Assessment will be conducted in support of a project to determine the
feasibility of modifying the elements of Trum Field in the Ball Square/Magoun Square area
of Somerville. Varying levels of positional accuracy will be needed for this project. With
regard to analyzing the park’s distance to bus stops and roads, accuracy of +/- 100 feet would
probably be sufficient. Positional accuracy of +/- 500 feet would be desirable for
determining the number of people who live within a certain distance of the field. Finally, a
very precise level of positional accuracy (+/- 1 feet or less) would be required when
determining what sorts of changes to make to the field (i. e., adding parking spaces,
expanding fields, modifying entrances, etc.).
2. Briefly discuss the three different road centerline data sets in terms of their positional
relation to each other (look at how far apart they are at different points using the
measure tool in ArcGIS, and if there is consistency in the differences. Include some
graphic examples to illustrate your points. Which data set would be best for your
project?
A comparison of MassGIS (red), Census 2000 Tiger lines (yellow) and Street Map USA
(green) road centerlines reveals some discrepancies. Census 2000 Tiger lines (yellow) and
MassGIS road centerlines (red) are most closely aligned. Of course, significant differences
occur with the greatest discrepancies falling within the 24-55 feet range. Street Map USA
(green) seems to be the least reliable of all, with a major road missing from the area down
and to the left from Trum Field. Street Map USA lines (green) diverge from MassGIS lines
(red) from anywhere from 31 to 70 feet, and they diverge from Tiger 2000 Census lines
(yellow) by 49 feet at points.
Both the Census 2000 Tiger lines (yellow) and StreetMap USA lines (green) include much more
information in their respective Attribute Tables (including street names and address ranges).
However, the MassGIS lines would be the best data set of the three to use (actually, I would
probably just want to pull the data from the city) for this project because of these lines most
accurately match up with the actual location of Trum Field as shown in the above 2005
orthophoto.
3. Do the same as above for the two hydrography (land use and parcel)layers.
For the project at hand, an analysis of hydrography layers would not be appropriate, so I have
compared MassGIS land use layers (shown above left, by itself) and Somerville parcel 2004 data
(shown above right, layered over MassGIS land use layer). These two layers essentially agree
with each other. The MassGIS layer identifying multi-family residential (orange), water-based
recreation (pink) and commercial (red) land use zones. The Somerville parcel data goes into
more detail, however. This layer includes parcels classified as: exempt (pink), industrial (green),
commercial (blue), residential (orange) and other (blue). It also shows isolated commercial
parcels which the MassGIS layer misses. The Somerville parcel data would be the best to use,
not only because of the greater detail in parcel level data and superior accuracy (I am not sure
why Trum Field would be considered a Water-Based Recreation area), but also because of the
level of detail found in the Attribute Table (including owner name, individual addresses and
mailing addresses). This could be useful in sending out some mail communication to
surrounding residents and businesses to elicit community feedback and ideas about the proposal
to modify the park.
4. Can you provide a quantitative assessment of positional accuracy for each of your data
layers (e.g., +/- 20 feet)? Why or why not? (If you have metadata, you might base an
answer on the source map's scale - here is the table of scales and the equivalent required
accuracy from the US National Map Accuracy Standard for Paper Maps - just remember
that when features are digitized from a paper map, additional errors are introduced! So
digitized data is never better than its paper source unless it has been through some
additional quality control process.
I am only able to provide a quantitative assessment of positional accuracy for the StreetMap
USA data layer. The scale of the source map used for this layer = 1/100,000. According to the
table of scales from the US National Map Accuracy Standard for Paper Maps, this layer would
have accuracy no better than +/- 166.67 feet, but would very likely have additional error. A
quantitative assessment of positional accuracy was not possible for the other layers because of
the lack of complete metadata.
5. Give a qualitative assessment of positional accuracy of each of the four optional layers
relative to the other layers (e.g., do streets run through buildings? are schools in the
correct location along a road?).
The four optional layers used for this analysis are: MassGIS MBTA Bus Stop Points, MassGIS
Building Footprints, MassGIS Open Space shape file and Census Environmental Justice Polygon
file. The bus stop points are essentially in line with all three road layers, but they match up best
with the MassGIS road centerlines (red).
The same can also be said for the building footprints layer, which show no discrepancy with the
MassGIS road centerlines (red), but have Census 2000 Tiger lines (yellow) and StreetMap USA
road centerlines (green) running through them at certain points.
The MassGIS Open Space layer displays gross discrepancies to the Census 2000 Tiger lines
(yellow) and StreetMap USA lines (green). And, the building footprints mostly align with the
buildings photographed by 2005 orthophotos- discrepancies may have more to do with changes
over time than actual inaccuracies.
6. Are these optional layers appropriate for your project in terms of their positional
accuracy?
The optional layers are appropriate for this project in terms of their positional accuracy. The bus
stop points provide an indication of the number of public transportation nodes in the immediate
vicinity of the park. The building footprints layer gives a very clear indication of the limits to
expansion which proposals for Trum Field might face. The EJ Polygon layer also gives an
indication of population characteristics in the immediate vicinity of the park. Areas with high
minority populations (purple), high foreign-born populations (yellow) and both high minority
and foreign-born populations (light blue) immediately surround Trum Field. This information
might inform the ways in which community residents are engaged, or what suggestions planners
might make to improve the park.
7. Completeness: Is each data set complete? (Does it cover the area question, are all
relevant features present, and is the attribute information complete for all features?)
The MBTA Bus Stop points are complete with the stop name and stop ID. The area in question
is covered comprehensively and all relevant features are present, but XY coordinates for each
stop are missing. The MassGIS road centerlines (red) cover the area in question, but street
names and address ranges are not present. The Census 2000 TIGER road centerlines (yellow)
and StreetMap USA centerlines (green) do not match up with the 2005 orthophotos very well,
but they include much more attribute table data than do the MassGIS lines (including street
names, address ranges and speed limits and elevation). The building footprint layer is quite
complete, but it lacks almost all relevant attribute data. The Somerville 2004 Parcel data
includes detailed attribute data and is mostly complete. The NLCD land cover land use layer
includes broad swaths of information covering the land use designation over a number of years,
but this layer lacks the detailed attribute data at the parcel-level. Finally, the 2000 Census EJ
Poly covers the area in question and includes such things as area census tract or block group total
population and relevant subset populations.
8. Currency: Are the data up to date? How do you know the answer to this?
The data display a range of currency. Where available, the currency of the data is indicated by
the Metadata under the heading: “Time period for which the data is relevant.” MBTA Bus Stops
are current as of May 2007, StreetMap USA lines are current as of 2000, MassGIS building
footprints are current from April to June 2002, and MassGIS Open Space layer as of July 2008. I
assume that the Somerville Parcel 2004 layer are current as of the end of 2003 or early 2004 and
that the 2005 orthophotos are current as of late 2004 or early 2005. However, in these latter
cases, I am merely making an assumption based on the title of the folders in which they were
found.
9. Attribute accuracy: provide a qualitative assessment of attribute accuracy for critical
attribute items (e.g., land use codes, street names and address ranges, school names, etc).
How adequate is the attribute information for your project needs?
The Somerville Parcel 2004 data layer land use codes referenced above right and in Question #3
are reasonably accurate for the task at hand. The attribute table above left provide a small
example of the addresses and other information displays a level of detail that would help with
making contact to those residents who would be most directly impacted by changes to the park.
As mentioned above, the attribute information available for the MassGIS road centerlines is not
sufficient for the needs of this project. It lacks address ranges and street names. So, for this
project, a different data layer put together by the City of Somerville (if available) would probably
work best. Other layers with much less level of detail, like the MBTA bus stops, would still be
adequate since a simple calculation of the number of stops within a certain radius of the park
would suit the needs of the project. Considering Trum Field’s location near the
Medford/Somerville line, it would also be necessary to gather parcel level data from the City of
Medford or MassGIS, if available.