Caitlin Bettisworth
Data Quality Assignment
The purpose of this assignment is to assess the quality of possible data layers that may be
used in a larger project. It is important when using GIS methodology to assess the quality
of each data layer used. Each source of data sets or data layers may collect data
differently or even have a different way of coding or drawing the data, which can lead to
discrepancies or problems when mapping. Thus, smaller projects like this are useful in
determining which data sets to use to best fit the scope of a given project. This
assignment will help in choosing the best datasets to use for a larger project, which aims
to tract artistic culture and cultural vibrancy using GIS methods. This project will look at
discrepancies in water boundaries, street centerlines, county boundary lines, open space,
building footprints, and geocoded event venue addresses.
Water Boundary Lines
In comparing water boundary lines from the Census to the BRA one can see clear
differences. The BRA layer is in blue and expands out past the Census water boundary. It
also seems to be much more accurate and detailed in some of the rivers and streams than
the Census water boundary. This is to be expected as one would expect information from
the city to be more accurate and detailed as the mapping area is less. In this case, the
BRA layer is at a scale of 1:276,198 whereas the scale from the Census 1:381,980. Either
of these would work in terms of the larger project; however, the BRA layer is more
preferred, as it is more accurate and detailed. In general, the project does not need the
water to be very accurate as the project is not analyzing anything with water, but if more
accuracy were possible, in this case, it would be preferred.
Water Boundary Lines: BRA layer in light blue, Census layer in light pink, the darker blueish purple is the
overlap between the data layers. The background World Imagery data is from ESRI and others expressed
on the bottom right hand section of the below map.
Street Centerlines
The Census Tiger Tracts from 2010 are compared against the newer version from 2013. It
appears that there are not many discrepancies between the two centerline layers. Census
Tiger Tracts 2010 was chosen because that is currently what is being used to geocode
venue and organization locations. However, it seems that once zoomed in enough there is
about a 2.5-3.0 meter difference in some of the tracts, with the newer census tract being
closer to the center of roads when comparing to the visual imagery. However, for the
purpose of the larger project either one of the layers would work. As it can be seen below
using the 2010 census tiger tracts produces accurate enough points for analysis, therefore,
either could be used. If the 2013 Census tiger tracts contain the same results in terms of
venue placement or even more accurate results they would be preferred.
Street Centerlines: The red line is from the 2013 Census Tiger Tract. The green line is from the 2010
Census Tiger Tract. This shows about a 2.5-meter difference in this section of tract, about the largest
difference found in Boston. The background World Imagery data is from ESRI and others expressed on the
bottom right hand section of the below map.
City Boundary Lines
Two data layers were looked at to determine a more accurate city boundary line.
However, MassGIS only provides a county boundary line whereas the BRA has only a
Boston City boundary line. Overall, these two can be compared, but they are looking at
two separate boundary lines. The BRA layer has a scale of 1:199,200 and seems fairly
accurate at encompassing all of Boston. The MassGIS layer has a scale of 1:2,556,505
and maps all of the towns in Massachusetts. For the most part both layers seem pretty
accurate in what they are mapping. This is no metadata for the BRA layer, but it can be
found that the MassGIS layer was created based on an Atlas that was published in the
early 1990’s. This is older than one would like, but as of now that is the best that can be
found. Near the docks both layers seem to be inaccurate. However, for the purposes of
the larger project a scale of 1:8,596 would be too large, and therefore, not necessary. For
the purpose of the larger project the MassGIS layer may be better because it encompasses
the county as well as the surrounding counties that will be used. Overall, it would easier
and better to use one source than multiple sources, if that one source is accurate and
precise enough for the project.
City Boundary Lines: The blue section represents the Boston boundary from the BRA and an overlap in
some of the county boundaries from MassGIS. The pink shows all of the county boundaries from MassGIS.
The background World Imagery data is from ESRI and others expressed on the bottom right hand section
of the below map.
City Boundary Lines 2: The blue area is from the BRA Boston boundary. The pink is from the MassGIS
Suffolk County boundary. The background World Imagery data is from ESRI and others expressed on the
bottom right hand section of the below map. This map shows that both data layers have some inaccuracies
when it comes to larger scales.
Open Space
When looking at the comparison of the MassGIS and BRA data layers on open space one
can see from the metadata that the MassGIS layer includes many categories when
defining open space: parks, forests, golf courses, playgrounds, wildlife sanctuaries,
conservation lands, water supply areas, cemeteries, school fields, and other open land. No
metadata is provided for the BRA data layer, however, based on similarities one may
assert that they have similar categories when defining open space. The MassGIS layer has
a very comprehensive list when looking at open space, it does however, seem to be
missing parts the BRA data layer picks up. On the image below the MassGIS layer is
pink and the BRA layer is blue when they overlap a dark pink or purple color is made.
Overall, these two datasets are pretty comprehensive and there are only a few places in
which one layer picks up open space and the other does not. One large difference
between the two data layers is that the BRA data layer does not include water in open
space areas. When zoomed in to an intersection or neighborhood level it also appears that
the BRA layer fits the open spaces a bit better. This shows that the BRA data layer is a bit
more detailed than the MassGIS data layer, which is to be expected due to the sizes of
each map (the BRA map has a scale of about 1:222,778 and the MassGIS map has a scale
of about 1:2,556,3000). Overall, for the larger project each of these would be acceptable
in showing open space. The MassGIS data layer although a bit less detailed may be
preferred to the BRA data layer in that when looking outside Boston it would be better to
have one source for open space than multiple town/city sources.
Open Space; This image shows the differences between the blue BRA layer and the pink MassGIS layer.
The lighter pink shows the areas the two that overlap. The background World Imagery data is from ESRI
and others expressed on the bottom right hand section of the below map.
Buildings
Buildings will not necessarily be used in the larger project; however, a quality assessment
on the buildings will help in narrowing down which data sources might be best. In
comparing the building outlines between MassGIS and DOIT one finds that MassGIS is
actually more accurate and shows slightly more buildings then the DOIT layer. In the
below image the DOIT data layer is in blue and the MassGIS layer is in pink. This is
actually surprising in that one would assume that the MassGIS layer would cover more
area and therefore would be less precise. This is not the case, actually the MassGIS layer
(1:199,655) and the DOIT layer (1:199,454) cover about the same map scale. In terms of
metadata both layers are lacking. The DOIT layer does not have any metadata, whereas
the MassGIS layer has a very broad description of what they consider a building. This is
because of how they created the layer using Light Detection And Ranging data from
2002-2011. By comparing the light data with ortho images from 2011-2012 they were
able to create the layer.
Buildings: The blue buildings are from the DOIT layer and the pink buildings are from the MassGIS layer.
This image shows that some buildings are mapped by MassGIS and not DOIT. The background World
Imagery data is from ESRI and others expressed on the bottom right hand section of the below map.
Events
During the larger project individual events and organizations will need to be geocoded
and mapped. The purpose of this section is to analyze six actual venue locations to make
sure the locations geocoded in arcmap using a MassGIS census tract locator provides the
correct locations. Based on where the locations are actually located the points given by
arcmap are correct. When zoomed in each location is mapped right out front of the
building, which for the purposes of the larger project is accurate enough. Other layers or
ways to geocode the venues might put the locations at the front door, but for the purposes
of the larger project on the street or even near the building will be sufficient as pin points
are helpful but not necessary.
Events: This image shows point event venue plotted at street level. This is not necessary for the larger
project, but it is good to show the 2010 Census Tiger Tract geocoded the venue in the correct location. The
background World Imagery data is from ESRI and others expressed on the bottom right hand section of the
below map.
Events 2: This image shows all six of the example event venues at a larger scale than will be needed. The
background World Imagery data is from ESRI and others expressed on the bottom right hand section of the
below map.