Jay Monty
10/22/10
GIS Assignment 4
Creating a Visitor Guide for Ipswich, Massachusetts
The town of Ipswich, Massachusetts, located approximately 30 miles north of Boston, is a small
town of approximately 12,000 residents. Each summer, a significant number of tourists visit the
many small towns in this area to visit the scenic beaches, historic sites, and enjoy the ample
hiking, canoeing and other outdoor activities available in the area. Many of these visitors arrive
by train from Boston, and the town of Ipswich would like to compile a visitor’s guide for the
downtown containing information about the many amenities that are a short walking distance
from the train station. As a preliminary task, a summer intern working in the town hall has been
assigned to compiling lists of these amenities as well as their location in order to create a
brochure map. He/she is hoping to use GIS as a means to compiling data so long as it is accurate
and complete. Positional accuracy is important, but only at a conceptual level. A 20 or 30 foot
discrepancy would not compromise the effectiveness of the brochure.
Two layers were used to compare accuracy for the streets around downtown Ipswich. In red, is
the layer provided by Mass EOT, while in green are the census TIGER streets. We can see very
quickly that the TIGER streets are grossly inaccurate and simplified. Little or no attempt was
made to add curvature to the lines where a curve exists in a roadway. Multi-legged intersections
are depicted by the TIGER file as simple T-intersections. However, while the EOT line work is
decidedly more accurate, it is often incomplete. In the top-left portion of the aerial photo shown
below, are two cul-de-sacs running southwest to northeast. While these are depicted inaccurately
in the TIGER file, the EOT file does not depict them at all. Conversely, if we look on the right
hand side of the aerial photo, we see at least two streets which are depicted by EOT but not
TIGER. A closer inspection suggests that these may not be streets, but private drives. Similarly,
EOT also shows the MBTA’s Newburyport Commuter Rail tracks as a street (albeit broken at
grade crossings) rather than on a separate layer for railroads.
Interestingly, if we compare the two street layers shown above to the Street Map USA layer we
find that there is a perfect match between Street Map USA and the Census TIGER data layer as
shown below.
For our project, the EOT file is clearly preferable. Though it is not as complete as the TIGER
file, all of the streets one would want to include on a brochure are clearly and accurately
displayed. This layer would be a good starting place to create a printable map in a brochure.
Hydrography: Upon attempting to determine the accuracy of the hydrography, it was found that
the TIGER hydro layer does not depict the Ipswich River, which is surprising because it is
perhaps the most important fresh-water resource in Essex County. The MassGIS layer is only
partially accurate and it appears that its depiction of the river is approximately 60 feet south of its
actual location where it turns east and crosses under Central Street. One interesting feature that it
does show is the location of a dam, displayed as a perpendicular line across the river.
As a comparison, when overlaying the StreetMap USA on top of the aerial imagery and
MassGIS layers, we see that the StreetMap layer, while unique, contains the same location error
in the vicinity of the Central Street Bridge.
Open Space:
At least in the downtown area, the areas defined by MassGIS as open space (green) are
somewhat inaccurate.
It appears that several of the areas depicted as open space are at least partially, if not entirely
privately owned and developed. One building downtown (top left photo) and the area along the
riverfront (bottom photo) appear to be private businesses, while a church occupies at least part of
the area listed as open space on the hill just above downtown (top right photo). One explanation
could be that these properties are in fact owned by the town, or, in the case of the church, at least
part of the property is publicly accessible. Still, whether this should be classified as open space is
questionable.
Geo-coding – Restaurants:
Though downtown Ipswich does not have a vast number of restaurants, the handful it does have
are geo-coded relatively well. Because the geo-coding is based on the TIGER streets layer,
which we know to be somewhat inaccurate, the locations are not perfect. Additionally, there are
a couple of restaurants which are missing, but overall, the locations are relatively accurate as
shown below.
Public Transportation:
Ipswich has limited public transportation options. The MBTA’s commuter rail line from Boston
to Newburyport is the only year-round public transportation service in the town, though a
seasonal bus service runs from the commuter rail station to Crane Beach and the nearby town of
Essex. The location of the commuter rail facility is accurately depicted below including the
station and parking lot.
Public Buildings:
There are several public buildings located in the downtown area including the fire department,
police department, town hall and town library. These are all accurately located by GIS with
exception of one of the libraries. GIS appears to count school libraries independently of schools.
The town hall now occupies a former school which closed several years ago, yet the school’s
library is still listed on GIS.
For most of the layers used, a true quantitative assessment of positional accuracy is not possible.
For example, the metadata for the open space layer states that data collected for open space areas
not controlled by the Massachusetts Executive Office of Energy and Environmental Affairs is
collected by volunteers, and thus the accuracy of the data is variable (MassGIS, 2008). Similarly,
for the positions of public buildings such as libraries, fire stations etc, the metadata states that
these were located based on a combination of geo-coding from city and town records, and then
refining the actual locations using aerial imagery. As with the geo-coding of restaurants shown
earlier in this exercise, it is difficult to state a positional accuracy, and as such, MassGIS does not
give one for these features.
Two layers which do provide a stated level of positional accuracy are the MBTA parking lots
and commuter rail lines. The metadata for the parking lots states that they were obtained from
1:5000 scale color orthophoto imagery. The exact locations were further refined by using
descriptions listed on the MBTA website (MassGIS, 2006). The 1:5000 scale source data implies
that the positional accuracy of the orthophoto imagery is +/- 13.33 feet. The use of the MBTA
website to find the exact locations should further increase the accuracy, though given the large
size of most parking lots, this is probably unnecessary. Likewise, the commuter rail line itself
was also based on a combination of topographical maps and orthophoto imagery varying in scale
from 1:5000, to 1:100,000. Thus its accuracy should also be within approximately 13 to 15 feet.
As for the base layers of streets and water, the Hydro 25k layer used for the water is based on a
map scale of 1:25,000. The national map accuracy standard for this scale is +/- 40 feet. As
discussed earlier, the location of portions of the Ipswich River appeared to be incorrect by
approximately 50 – 60 feet, which would fall just outside of the accuracy standard. Similar to
the commuter rail line-work, the EOT roads layer, the more accurate of the roads layers was
compiled using 1:5000 scale color orthophoto imagery.
The four optional layers (open space, restaurants, public transportation and public buildings) are
located fairly accurately, with a couple of exceptions described earlier. In fact, they are probably
more accurately located than the streets and waterways. The TIGER streets, being particularly
inaccurate, did create inaccuracies in the geo-coding where objects were shifted slightly from
their actual location. Because Ipswich does not appear to have building outlines in the MassGIS
dataset, and all facilities are listed only as points, the potential for inaccuracy is somewhat
reduced, or at least made less obvious. The major question mark surrounds the open space layer
since several parcels in the downtown area appear to clearly not be open space and as the
metadata stated, the data for this layer was compiled by volunteers and has varying accuracy.
For the most part, the positional accuracy of the four optional layers is appropriate, though some
refinement for the geo-coded data would be necessary. Most of the accuracy of the optional
layers was obtained through the use of the orthophotos (public buildings, MBTA parking lots
etc.) and the same would need to be done for the restaurant locations obtained through geocoding. The open space layer, though perhaps slightly inaccurate, still provides a clear
representation of the locations of open space in the downtown.
Some of the data sets are not complete. For example, the EOT roads layer is clearly missing
some minor roadways, and the fact that the TIGER hydro layer does not include the Ipswich
River is very significant. Several restaurants were not available through geo-coding, though it’s
possible that they could be found under a different class of data. However, the most crucial data,
such as the public safety buildings appears to be complete and accurate. As stated earlier, it
appears care was taken in compiling this data by making use of town records, and ortho-imagery
to ensure the completeness of information about these facilities.
All of the data appears to be up to date. The metadata for each layer gives the date of creation as
well as any updates. All of the layers used were created in the mid 2000’s and many, such as the
public safety buildings are listed as being updated “as needed” to ensure that the information
about these important facilities is kept current. Most have been updated within the last 2-3 years.
Only two layers did not clearly indicate their currency, and these were the Census TIGER files.
However, a safe assumption is that these layers were created around the time of the 2000 census
or slightly before.
The accuracy of the attributes appears to be relatively complete with only minor discrepancies.
Some zip codes are incomplete or unlisted for the libraries in Ipswich as well as the fire stations.
The attribute information for the MBTA parking lot seems complete and accurate, as does the
information for the geo-coded restaurants.