Katie Aasland
3/7/2012
UEP 232: Intro to GIS
Assignment 4 – GIS Data Quality Assessment
Project Description
For my final project, I want to examine the location of schools and after school programs in New
Orleans. I want to see if schools and programs are evenly spread out throughout the city, or if
they cluster in certain areas. Also, do after school programs exist near schools or would the youth
be required to take transportation to get there? Because most parents work during the day, I also
want to look at whether there are bus lines that run in the areas of the schools and programs that
would be able to transport youth. However, I am unfamiliar with the New Orleans area and am
unsure how reliable the layers will be, so I want to do a test run of geocoding and mapping the
layers with an area in which I am familiar. I chose to examine the neighborhood surrounding the
Arthur D Healey Elementary School in Somerville, MA.
Positional accuracy is only relatively important for roads, hydrography, and parcels. School, bus
stop, and after school program information must be more accurate because youth would be
traveling unaccompanied from one location to the next. In more dangerous neighborhoods (like
the ninth ward in New Orleans), the amount of time a youth is outside unsupervised should be
minimized. I also want to be able to capture all schools and programs so schools, bus stops, and
after school programs must be complete and up-to-date.
Roads
Hydrography
Schools
Parcels
Bus Stops
After School
Programs
Positional
Accuracy
+/- 10 feet
+/- 30 feet
+/- 6 feet
+/- 10 feet
+/- 10 feet
+/- 6 feet
Required
Attributes
Street names
Completeness
Time Period
Important
Current
School names
Important
Current
Building names
Important
Important
Current
Current
Road Centerlines
I examined the MassDOT (red) and the TIGER (green) road centerlines for the neighborhood
surrounding the Healey. These centerlines were almost exactly the same; the only difference is at
the intersection highlighted by the white box (Figure 1). The lines are even the same where they
appear off-center. However, when I zoom in closer to street level, this issue is corrected.
Because the two files are so similar, either file would make sense to use due to positional
accuracy. However, the MassDOT file has more specific information on road classifications, but
it is from 2009. The TIGER file is from 2010 and it has street names included in its Attribute
Table. Therefore, I would choose to use the TIGER file.
Figure 1. Road Centerlines
Hydrography
The two hydrography layers I examined were from MassDEP on the MassGIS website and
TIGER. MassDEP underestimates the edges of where the water is; in one section is it about 34
feet short of shore. TIGER seems to overestimate where the water is, using angular lines.
However, in one section TIGER is also about 34 feet off from shore. The MassDEP file is an
enhanced version of an older geological survey hydrography layer, so it makes me wonder if it is
actually marking data older than 2009 then. This layer has an extremely detailed categorical
system for the bodies of water, which may be more useful in a different project. I was unable to
find information on the development of the TIGER file, but it is able to provide latitude and
longitude for the bodies of water as well as names. If I were to use a hydrography layer, again I
would go with the TIGER file.
Figure 2a. MassDEP Hydrography
Figure 2b. TIGER Hydrography
Schools
To examine schools, I pulled the layer from MassGIS since it was easily available (red schools).
I then went to the Somerville Public Schools website and got the addresses for all the schools in
the city. I geocoded using the North American Geocoding Service in ArcMap10, giving me the
yellow schools.
Positional Accuracy
Even though the markers for all schools were all over 100 feet from each other, they both still
indicated the school. Looking closer at the Healey School (Figure 3) shows that MassGIS places
indicators at the center of the school, while Geocoding marks the front entrance. Since I am
examining the location of the school to other buildings, either dataset would be sufficient.
Figure 3. Healey School
Completeness
The MassGIS dataset documented all of the Somerville schools using address-matching with the
Department of Education and refining addresses using Internet map sources and phone calls.
With Geocoding, all 8 Somerville schools were completely matched.
Currency
MassGIS is current to 2009. The Somerville Public Schools website says it was last updated this
month, so if a school were to have closed, I would assume it would not be on the website.
Attribute Accuracy
The MassGIS attribute table contains extensive information on the school including grades at the
school and school type (charter, public, private, etc.). The NAGS attribute table also contained
XY coordinates as well as longitude and latitude, which are not as helpful as the information in
MassGIS.
Parcels
I compared the Land Use file from MassGIS and the Parcel file from the Somerville City data.
This was a bit challenging because I was not comparing identical type files, but it was helpful to
see what the difference is. The bright green represents Commercial areas and light blue is
Residential. The biggest difference between the two is the periwinkle area, which I will discuss
below.
Positional Accuracy
The MassGIS set is problematic because it does not show parcel boundaries or roads because it is
just examining land use (Figure 4a). This makes it hard to orient oneself to what they are looking
at when examining a small area. The Somerville set lines up with roads and buildings (Figure
4b), distinguishing each individual parcel.
Completeness
The MassGIS dataset is complete and has no unknown information. The Somerville dataset does
have missing parcel information, making it incomplete. If I needed to find the missing
information, I would need to check other sources.
Currency
The Somerville data set is 4-years more current, which would make it the better choice to use.
This is especially true because of the economic events that have occurred between 2005 and
2009. The economic downturn may have closed businesses or changed the layout of the city like
in Rust Belt cities.
Figure 4a. MassGIS Land Use
Figure 4b. Somerville Parcels
Attribute Accuracy
The level of detail provided for the MassGIS dataset exceeds what would be included in this
project, but would be extremely helpful for others. The Somerville data is sufficient and does not
provide an overwhelming amount of information. It is also easier to read at a glance. However,
there is one area of discrepancy between the two: The Mystic Housing Authority. According to
MassGIS, this is just included as residential space. However, Somerville distinguishes it from the
surrounding housing by classifying it as “Exempt”. This places it in the same category as the
school next door. This is a difference that should be considered.
Bus Stops
I pulled data points from MassGIS about the MBTA bus stops (red) and then attempted to
geocode the names of the sites to create new points using the North American Geocoding
Service (yellow). However, the only stop this was able to match was the one at the intersection
(Figure 5).
Positional Accuracy
Both sets of points are placed in the middle of the street and are therefore about 10 feet from the
actual bus stops. This is because the stops were created using GPS on the buses, which would
have been in the road.
Completeness
The MBTA is the most complete layer. It is from the Transit Authority and marks all stops.
Currency
The MBTA file was created from information gathered in May 2007, indicating that it is not very
current. The Transit Authority could have added/removed stops or even bus lines altogether in
this time, which would make this layer inaccurate. However, I live right near here, so I know
these stops are still in use.
Figure 5. MBTA Bus Stops
Attribute Accuracy
The attribute table provides the Stop ID and direction (inbound or outbound), if that were
needed. However, it does not provide information coordinate or latitude information.
Afterschool Programs
Because Reference USA and MassGIS do not provide information on after school programs, I
pulled a list of after school programs off of the Somerville Public Schools website and geocoded
the addresses using North American Geocoding Service.
Positional Accuracy
The closest program to the Healey School is the Mystic Learning Center in the Mystic projects.
The marker is in the street in front of the correct building, but not near the entrance for the
program. It’s over 100 feet off (Figure 6)! However, because it identifies the correct building,
that is all I need.
Completeness
I created this dataset from the Somerville Public School website, finding the names and
addresses of all the schools in the city. However, I know there are other programs in this area
that were not included on the list, so I would need to check multiple sources to find addresses.
Currency
The Somerville Public School website says it was updated at the end of January this year so it is
current.
Figure 6. Mystic Learning Center
Attribute Accuracy
After geocoding, the correct buildings are indicated and I now have XY coordinates as well as
latitude and longitude information.