1. Provide one-paragraph description of the project you are using as a benchmark to assess the data and what positional accuracy it will require (or what is good enough - think about how far off the position could be and still work for the project needs).
This project is developed on behalf of hospital administrators at the Arbour Hospital System in Boston, which is the the largest private mental health system in Massachusetts. Hospital administrators are interested in encouraging exercise and healthy living programs as 1) part of their treatment for patients and 2) to encourage general health and well-being among employees. Two ideas that particularly interest them are the accessibility of nearby parks and public transportation for patients and employees. Administrators believe that if more employees commute by public transportation they will get a half to full mile of walking in every day. They also want to encourage employees to use nearby parks for exercise during lunch and/or breaks. Thus, the goal of this report is to analyze accessibility of nearby parks and subway stops.
1. Briefly discuss the three different road centerline data sets in terms of their positional relation to each other (look at how far apart they are at different points using the measure tool in ArcGIS, and if there is consistency in the differences. Include some graphic examples to illustrate your points. Which data set would be best for your project?
Streetmaps scale is appropriate for maps at 1:50,000. Tiger Census scale is appropriate for maps at 1:100,000. Due to the large scales, both Streetmaps and Tiger Census have inaccuracies at the level for this project. They often show roads cutting through buildings or along sidewalks when viewed at this project's detailed scale. For example, the point where Robinwood Ave (the street on which Arbour Hospital is located) intersects Centre Street is approximately 70 feet south of the actual location in both the Tiger and Streetmap line files. BWSC, on the other hand, is considerably more detailed, and generally follows the street edge exactly. This is to be expected, especially considering the scope of work each institution covers. Tiger Census and Streetmaps are providing maps for the country. BWSC will focus only on the city of Boston.
Lastly, for this project Streetmaps and BWSC datalayers will be used. Streetmaps has additional attribute information - like road names - which is very useful, though the BWSC street edge will be used to measure distances.
2. Do the same as above for the two hydrography layers.
I used two hydrography layers within my area of interest: one from MassGIS and the other from the Boston Redevelopment Authority. Similar to the road layers described above, the municipal data provided by the BRA is more accurate than MassGIS's state level data. Interestingly, MassGIS provides an "enhanced" version of USGS hydrographic data - which is appropriate for maps at 1:25,000 scale. Metadata for the BRA data does not indicate appropriate scales; however, it is evident from the Boston orthophotos that the BRA hydropgraphy closely follows natural water features (certainly closer than the MassGIS dataset). For example, in the northeastern corner of Jamaica Pond, the BRA shapefile borders the edge of the pond as depicted by the orthophoto exactly, whereas the MassGIS data is offset by approximately 50 feet at the same point.
For the purposes of this project, the Hydrography shapefile from the BRA will be used.
3. Can you provide a quantitative assessment of positional accuracy for each of your data layers (e.g., +/- 20 feet)? Why or why not?
Yes, for most of the layers I can. The FGDC, USGS and other national cartographic organizations follow national map accuracy standards based on the publication scale (i.e. 1:20,000). Specific points on the map are tested and the relative accuracy of their position are compared to well-defined points on 1) more accurate maps or 2) real world points. Because I know the recommended scale of most of my map layers (and with a phone call could sort out the recommended scales for the rest), I could calculate the positional accuracy of specific points for each datalayer. See www.fdgc.gov for more.
4. Give a qualitative assessment of positional accuracy of each of the four optional layers relative to the other layers (e.g., do streets run through buildings? are schools in the correct location along a road?).
The relative location of the hospital, buildings, subway lines, rail lines, open space and subway stations (radius in pink and blue) are all good in this map. As noted above, this is expected because five of the six layers above were taken from city agencies, which have focused on mapping Boston attributes only. All points and lines correspond exactly with the orthophoto. Only the hospital location is taken from MassGIS, which is mapped on a scale that may be less appropriate for city level analysis. Nonetheless, there are some small errors in the layers. While the positions of the building layers are good (as compared to the orthophoto), some small buildings have not been mapped. See for example the two buildings in the white square at the upper left hand corner of the map, which do not appear in the buildings layer.
5. Are these optional layers appropriate for your project in terms of their positional accuracy?
Yes, they are detailed and positionally accurate. Before proceeding, however, it would be worthwhile to calculate the percentage of error in feet of distances between known points. Moreover, it might be worth it to take a pedometer to the streets and compare a couple measurements with my map calculations.
6. Completeness: Is each data set complete? (Does it cover the area question, are all relevant features present, and is the attribute information complete for all features?)
As noted above, some datasets are incomplete (i.e. the building layer). In addition, the BWSC street edge is incomplete around the hospital parks lots and other back roads.
7. Currency: Are the data up to date? How do you know the answer to this?
The Metadata (in ArcCatalog) provides information on when the data was collected and geoprocessed. Unfortunately, for most of the BWSC and BRA data, the time period from which the data was collected is "unknown." In light of this, I would put call BRA and BWSC to determine the timeperiod.
8. Attribute accuracy: provide a qualitative assessment of attribute accuracy for critical attribute items (e.g., land use codes, street names and address ranges, school names, etc). How adequate is the attribute information for your project needs?
Hospital: source is MassGIS; as a point, it accurately marks the hospital.
Buildings: source is BWSC; as described above, it is poisitionally accurate though in a few cases buildings are not marked as they appear on the orthophoto.
Subway Lines: source is BWSC; the subway lines are positionally accurate.
Rail Lines: source is BWSC; the rail lines are positionally accurate and in most cases also correspond to the subway lines. Of course, a rail line (separate from subway) tracks the street to the west of the hospital.
Open Space: source is BRA; the open space layer is positionally accurate and includes parks, baseball fields and other recreational areas.
Subway stations: source is BRA; subway stations include a .25 and .5 radius (in yellow and pink, respectively) indicating the distance to the station.
I believe the attribute information is adequate for my project.