Jeanette Rebecchi
GIS Assignment 4
October 22, 2010
1. Project Description
For my UEP Urban Design class, my team and I are preparing a report regarding the
potential redevelopment of the vacant Star Market site in Somerville, Massachusetts. This site
located on the corner of Broadway and Temple Street in Winter Hill is currently the second
largest vacant parcel in Somerville and the community has strong preferences as to what they
want to see there in the future (mainly another grocery store). This project will primarily
examine using GIS the spatial location of other potential competing retail entities that could
interfere with the future success of the site. Looking at the potential uses that could be placed at
this location is a key aspect to conducting a full analysis of how this site could be developed in
the future. The positional accuracy of the geocoded data is important primarily down to the block
level in this case. However, the real test is to ensure that all the right data is displayed on the map
and that none are missing.
2. Road Centerline Layer Analysis
In order to properly design the Star Market site it is necessary to know what roads are in the
vicinity of the site to help determine vehicle circulation patterns. In Image 1, the red arrows
represent all streets that are displayed in GIS but do not actually exist in real life. You can tell by
examining the orthophoto underneath. The orange arrows represent streets that do exist but are
misaligned compared to their true position.
Overall the City of Somerville street centerlines displayed in green are most accurate, falling
directly in the center of the displayed orthophoto streets. The Census 2000 TIGER street lines in
blue and the StreetMap USA lines in orange overlay each other half the time, though they are on
average thirty feet from the ortho photo/Somerville street centerlines. When they don’t overlay
each other, the StreetMap USA layer seems to be the furthest away from the true street
centerline, followed by the Census 2000 TIGER road lines in all cases. Overall, I would choose
the Somerville street lines over the other two since they are the most postionally accurate, though
all included “phantom streets”.
Image 1: Street Centerlines
3. Hydrography Layer Analysis
While hydrography data is not necessary for the scope of my project, a reference map would be nice
to include in the final report. When comparing the Census TIGER Hydro layer with the one provided
by Mass GIS, the later featured in light blue in Image 2 is far more positionally accurate. The TIGER
Hydro layer in dark blue is very roughly drawn compared to the level of detail and curvature
provided by Mass GIS, often missing the true water boundary by about 160 feet. I would prefer to
use the Mass GIS data in this case also because they provide Mystic River data as well.
Image 2: Hydrography
4. Positional Accuracy
City of Somerville Road Centerlines: Unknown, no metadata was made available from the City
of Somerville.
Census TIGER Road Center Lines: According the Census website, all source materials used to
create the TIGER road layer are no better than 1:100,000 scale and are not suitable for accurate
measuring since the lines can be off by +/- 166.67 feet
StreetMap USA: Unknown, no metadata was attached to the downloaded file. Could not find
additional information on their website.
Census TIGER Hydropgraphy: A 1:100,000 scale standard has been used since the 1990 census
which translates to a positional accuracy of +/- 166.67 feet.
Mass GIS Hydrography (25k): Metadata states a 1:25,000 accuracy level which translates to +/40 feet.
Mass GIS SCHOOLS_PT layer: The data was created from 1:5,000 ortho photos or 1:25,000
digital topographic imagery giving a positional accuracy range of +/- 14 - 40 feet. However
internet map resources, street atlas, and phone interviews were also used to further refine
locations.
Mass GIS LIBRARIES_PT layer: This layer was created via a geocoding method so positional
accuracy on orthophotos cannot be ensured. Any libraries that were mismatched during the
geocoding process were located via online research and phone calls to library staff. Any library
listed as part of a k-12 school was compared to the SCHOOLS_PT layer above and shifted to
coincide with the location of the school points in the above mentioned layer.
Grocery and Convenience Store Layers: These geocoded layers were obtained from Reference
USA with information regarding street address and locations. The data points were geocoded
with the Census 2000 TIGER street lines which already do not match the orthophoto street lines
and buildings. Using the TIGER 1:100,000 scale standard this information could be off by +/166.67 feet.
5/6.
Qualitative assessment of positional accuracy and appropriateness for project
The Mass GIS SCHOOLS_PT layer includes all the preschool through high school schools in the
state of Massachusetts including all public, private, charter, collaborative programs, and
approved special education facilities. There seems to be only one school (circled in Image 3) that
does not fall on any particular building. Additionally, I don’t know why in two cases there are
two school data points represented by flagged buildings on one building complex. There’s only
one actual school at that location but two data points. Overall this information is useful to show
that the Star Market site would not benefit from having a school built on it since there are already
several schools in a half mile radius.
Image 3: Somerville Schools
The Mass GIS LIBRARIES_PT layer includes all libraries currently registered with the Massachusetts
Board of Library Commissioners. For the most part the libraries fall on a building except for one shown
below which was off from the school library by about 200 feet and is placed on a park (see Image 4).
Image 4: Somerville Public Libraries
I’m assuming it’s a duplicate point since there already is another library point on top of the school
behind it. Using the library layer by itself would be misleading in this project since in my mind a public
library is a place that can be used openly and freely by Somerville residents. Calling a school library a
public library is not entirely accurate since it is not open to the general public. Once the two layers are
displayed at the same time you can see that most Somerville libraries are within schools except for eight
stand alone locations. However, after going to the Somerville public libraries website there are only
three stand alone branches. This data would not be very useful for my project since I’m only concerned
about the location of standalone public libraries and whether one could be used as an anchor
development at the vacant Star Market site. This data layer displays many more libraries then actually
exists in real life making me doubt the accuracy of these points.
Image 5: Somerville Schools & Libraries
The Grocery and Convenience Store layers were geocoded against the US Census TIGER street lines
not the orthophoto, so all data points are away from the building by about thirty feet. Blue triangles
represent convenience stores, while the green stars are grocery stores. All points look like they belong to
a particular building expect the convenience store furthest to the top of Image 6. This point has been
positioned over a vacant site indicating that perhaps there was a store on that parcel at one point in time.
Overall, the precise position of these data points is not relevant to my project since I’m more considered
about the number of the competing food stores in the area. There is speculation that the vacant Star
Market store closed because of competition from Super Stop and Shop found to the right of Image 6. So
before we investigate whether or not to put another grocery store on this vacant site we need to figure
out how much competition there is in the area and how close other food stores are within the scope of a
few blocks.
Image 6: Winter Hill Area Grocery & Convenience Stores
7. Completeness
All physical features appear complete, though the Census 2000 TIGER Hydrography layer doesn’t
provide any shoreline information for the Mystic River which is the closest body of water to my site.
The schools and libraries layers are complete though overabundant in terms of providing too many
libraries. For the food store layers, a grocery store called Bedley’s Mini Mart is missing an address so it
could not be geocoded to the map. However, I know for a fact after visiting the Star Market site several
times, there are at least a dozen other small supermarkets along Broadway alone that did not appear on
the list provided by Reference USA. Therefore, I could not use this data to conduct a spatial survey of
potential competing food stores.
8. Currency
For both the Census 2000 TIGER layers, the information was taken from the 2000 census so they are
over ten years old now. This could mean that some roads have been realigned or shoreline water features
could have changed over the past decade. This information was obtained from the Mass GIS website
regarding census data layers.
The StreetMap USA does not say when the information was obtained.
For the following Mass GIS layers, I obtained the information regarding the currency of the data from
the metadata associated with the file. The Mass GIS SCHOOLS_PT layer was last updated from
information obtained by the Department of Education on December 28, 2007. In terms of schools this is
still very recent. The Mass GIS LIBRARIES_PT layer was last updated as of October 2004 so its a few
years old, but libraries don’t change that often. The Mass GIS hydrography layer was last updated in
October 2004.
For the Grocery and Convenience Store Layers a search of their website could not provide when the
information about Somerville grocery and convenience stores were obtained. All their website attributes
to data currency is that their data is “continuously updated from more than 5,000 public sources”. It is
impossible to know when or how this information was obtained, or if they captured all of it (which they
did not).
9. Attribute accuracy
In all attribute tables there is some human error concerning misspelling of names, missing information,
etc. Since the scope of my project primarily concerns whether something exists or not and where, the
accuracy of the actual naming does not matter.