Caitlin Dolan
Intro to GIS – UEP 232
Assignment 4
March 10, 2011
1.
Project Description
Next week at my work, The Possible Project (www.possibleproject.org), we are sending our youth on a
scavenger hunt around Cambridge. The teens will be split into teams of three and each group will be
given a clue sheet, including instructions and coded directions, to guide their scavenging. The clue
sheet will lead them to art dealers in various commercial centers of the city. We chose art galleries as
the focus of the scavenger hunt because our in-house business is arts based (digital imaging to be
exact) and we want to educate our youth on the competition and/or potential vendors. To create our
clue sheet we will be assessing the following data:
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Street center lines
Bodies of water
Open Spaces
Historic landmarks
Art galleries/dealers
While these youth are residents of Cambridge, they are not often in the areas where the scavenging
will take place. To ensure the safety of our youth we need accurate data, ideally +/- 20 feet. Our data
must be complete and as recent as possible. Having data from 2000 or later will be acceptable for the
streets, bodies of water, historic landmarks, and open spaces as these things change infrequently, if
ever. However, the data on art dealers must be within the past couple of years since retail stores have
high rates of turnover, especially in today’s economic climate. The attributes for these datasets must
include addresses (or coordinates), names, and in the case of the art dealers it would be ideal to have
contact information. There must be a very strong connectivity between all of the data layers’ points to
guide our youth accurately around the city. Many of these galleries are small and situated on narrow
roads, thus the more accurate the surrounding parks, lakes, statues, and streets they have to guide
them, the better.
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Caitlin Dolan
Intro to GIS – UEP 232
Assignment 4
March 10, 2011
2.
Street Center Lines
The image below is an aerial photo of an intersection in Harvard Square. I have chosen to compare
three sets of street center lines to determine which is the most accurate and best suited for our
scavenger hunt. The pink lines represent street data from the City of Cambridge, the orange lines are
street data from the Census TIGER lines, and the blue lines represent street data from StreetMap USA.
Focusing in on Mass. Ave. only the City of Cambridge line stays on track with the paved roadway, while
the street lines from TIGER and StreetMap both veer off and cross along the top of a church. The
TIGER line for Mass. Ave. is 70 feet off from the more accurate City of Cambridge line, while the
StreetMap road line is further still at 102 feet away. Looking down Church Street the City of Cambridge
streets also most accurately reflect the true location and shape of the street. The City of Cambridge
line for Church Street mimics the street’s actual curvature, while the TIGER line and StreetMap line
both inaccurately represent Church St. as a perfectly straight road. The displacement of roads by
TIGER and StreetMap are relatively consistent across the City of Cambridge.
The StreetMap USA metadata for this file, reveals that the data is at a scale of 1:100,000, has an
accuracy level of +/- 166.67 feet, is from the year 2000, and is based on a TIGER 2000 streets dataset
that was enhanced by ESRI and Teleatlas, thus accounting for the slight variations to the TIGER line
locations. There is limited metadata for the Census TIGER lines, however I was able to find out that it is
from the year 2000 an is at the same scale as StreetMap USA, 1:100,000, and thus at the same
accuracy, +/- 166.67 feet. There is no metadata available for the City of Cambridge’s street center lines
file, however I assume it has a larger scale since its higher accuracy is so clearly revealed in the image.
The City of Cambridge dataset appears to be most complete. As shown in the image, only the City of
Cambridge file includes a line for Peabody Street which is located to the right of Mass Ave., thus it is
clear that this dataset is the most complete. Based on all of the above, the City of Cambridge dataset is
best suited for this project because the youth must have highly accurate maps to guide their
scavenging. The attribute information for each of the datasets includes the key fields of name, and
address coordinates, i.e. left from xxx, and right from xxx. The City of Cambridge has much more
complete information within its attribute, particularly in the name field in comparison to the TIGER and
StreetMap’s attributes.
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Caitlin Dolan
Intro to GIS – UEP 232
Assignment 4
March 10, 2011
3.
Bodies of Water
There are many bodies of water in Cambridge that could help direct our youth. Most prominent is the
Charles River which is a mere 1,000 feet from the heart of Harvard Square. The image below includes
two data sets. The dark blue lines represent a hydrography file from the City of Cambridge, while the
light blue lines represent a hydrography file from Census TIGER data. The TIGER data is from the 2000
U.S. census, and produced at a scale of 1:25,000, which puts the accuracy at approximately +/- 40 feet.
Metadata is not available for the City of Cambridge’s hydro file, but based on the image it seems that it
must be at a larger scale than the TIGER data. The attribute information is quite similar and complete
for each of the datasets, so this does not separate out the two sets in any way.
The attribute information for the Cambridge hydro file states the year each item was collected. The
date of data collection for the hydro file ranges from 1995 to 2003. Despite the fact that many of the
Cambridge data is older than the TIGER data, I think the image below says it all. The dark blue lines
representing City of Cambridge data are quite accurate, following each curve of the river. By contrast
the light blue lines of the TIGER data are rigid, straight, and quite off target (113 feet!) from the aerial
photo’s depiction of the river. The displacement of the water bodies by the TIGER dataset holds to be
relatively consistent across the City of Cambridge. Assuming our scavenger hunt requires bodies of
water as a frame of reference, I am going to recommend that we use the City of Cambridge’s
hydrography dataset.
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Caitlin Dolan
Intro to GIS – UEP 232
Assignment 4
March 10, 2011
4.
a) Open Spaces, Historical Landmarks, and Art Dealers
The below map includes open space, historical landmarks, and art dealers. The first two datasets were
obtained from The City of Cambridge, while the art dealers data was downloaded from Reference USA.
There is no metadata available for the open space file, thus there is no way of knowing the data’s
positional accuracy. As I am very familiar with Harvard Square, I can confidently say that several of
these alleged “open spaces” are actually paved areas or buildings, thus it is quite misleading. On the
one hand, I don’t see any open spaces that are unmarked, but on the other hand there are several
areas inaccurately marked as open space. There are several empty entries in the open spaces attribute
table, which is particularly challenging in the name field. It’s always helpful to know the name of a city
park when using it as a directional reference. I do not think this dataset’s attribute information is
adequate enough for our project needs. Unnamed and inaccurately mapped open spaces have the
potential for really confusing and misdirecting our youth.
Similarly there is no metadata available for the historical landmarks data layer. The City of Cambridge
has a rich history, thus it is not surprising to see so many historical spots (marked with a pink circle).
That being said, I travel through this area on a daily basis and can attest that the vast majority of these
“historical landmarks” lack signage. Furthermore, many of these historical sites have been
redeveloped to such a degree that their former use, e.g. a house that is now a commercial building, is
not recognizable. That being said, this historical dataset is highly complete and comprehensive. It
seems to include every historical site that could possibly be conceived. The complete nature of this
dataset could also present a challenge for the youth on our scavenger hunt. Our youth, like most
people, will expect a “historical landmark” to be prominently marked with a plaque or statue and that
is not the case for the vast majority of these sites. Thus, we will have to weed out the less well known
historical sites, which lack signage. The attribute data appears to be accurate in terms of the two most
critical fields, address and site name. Also, none of these fields are blank which is reassuring. The
attribute data provided by the City of Cambridge is adequate for our scavenger hunt, but again we’ll
only be selecting a few historical sites for the youth to use as a directional reference.
Reference USA does not provide metadata, thus I cannot provide a quantitative assessment of the
positional accuracy. The one art dealer, MDF, that is located in this map frame is marked with a blue
circle. I am familiar with MDF and can say with certainty that this blue circle is not an accurate spatial
marker. Reference USA place the art dealer in the middle of a pedestrian walkway along a street that
is filled with shops. The store is actually located 103 feet away, as marked on the below map with a
red X. Reference USA does not provide a reference year for this data. There is a data quality page on
the Reference USA vouching for its own high standards: “Our database of 14 million U.S. businesses is
the most accurate and comprehensive in the industry… our databases are continuously updated from
more than 5,000 public sources.” It’s a nice sentiment but it doesn’t give me any hard facts to go on.
As such, I will have to visit the addresses that Reference USA provides for the 13 total Cambridge art
dealers to verify their accuracy myself. On second thought, my first step will be to actually call each of
the 13 art dealers as Reference USA provides this information. If I’m not able to connect with the art
galleries, then I will make a visit. The attribute data really does have everything that I’m looking for –
address, owner name, phone number, etc. The biggest downside to this data is that I cannot say with
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Caitlin Dolan
Intro to GIS – UEP 232
Assignment 4
March 10, 2011
100% certainty that the Reference USA dataset is complete or 100% accurate, unless I scour the whole
City, street by street myself.
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