Kendall Webster: Assignment 4

This project will demonstrate recreational opportunities available to pedestrians in the north harbor area of Nantucket Island. There are various data layers available to show recreational opportunities in this area: an open space polygon layer from MassGIS, and sports facilities, playground and sidewalks layers from the Town of Nantucket. If analyzed from the view of pedestrian users, positional accuracy of these data is important because inaccuracies in distance and location cost time and physical effort. Size of open space areas and playgrounds is important for recreational planning. Also, if a map of recreation opportunities for pedestrians was published by the Town of Nantucket, data accuracy would be necessary to achieve good professional quality. For publishing purposes, the position of recreational opportunities should not exceed 30 feet of their actual location.

Road Layers

The three road layers used in this project vary in degree of accuracy. These layers are: road centerlines from MassGIS, road centerlines from StreetMap USA and road centerlines from the Census TIGER data set. The MassGIS roads layer and the road centerlines from StreetMap USA are more accurate than the TIGER data. The MassGIS and StreetMap USA layers lie physically on top of the roads in the orthophoto, while the TIGER roads deviate by up to 90 feet from the roads in the aerial imagery. This disparity is depicted in Figure 1, the red lines are the MassGIS roads, the green lines are the StreetMap USA roads and the yellow lines are the TIGER roads. In the area within the orange square, the TIGER roads range between 60 – 90 feet from the roads in the aerial imagery. However there are points at which the TIGER roads do overlap with the roads in the aerial imagery. The orange arrow in Figure 1 indicates an example of this occurrence. The spatial inconsistency of the TIGER road layer is not consistent.

Figure 1. Positional accuracy of all road layers

However, the MassGIS and StreetMap USA road layers are not interchangeable in accuracy. In some instances, the MassGIS layer shows roads that the StreetMap USA layer does not; in other instances this relationship is reversed. Figure 2 depicts both situations, where the red lines are MassGIS roads and the green lines are StreetMap USA roads. In the red box, the StreetMap USA layer captures smaller side streets which the MassGIS roads do not. In the orange box, the MassGIS layer captures a significant side street that the StreetMap USA layer does not. For the purposes of this project, both StreetMap USA roads and MassGIS roads are acceptable.

Figure 2. Positional accuracy of StreetMap USA and MassGIS road layers

Hydrography Layers

The hydrography data tested for this project was obtained from MassGIS and from TIGER Census data. In this region there is only one solid water body, the Nantucket Harbor. The TIGER Census data does not capture this water body, MassGIS data does. Figure 3 depicts the Nantucket Harbor; the arrow indicates an area where the MassGIS layer deviates from the aerial image by 50 feet.

Figure 3. Positional accuracy of the MassGIS hydrography layer

An alternative data layer from the Town of Nantucket for hydrography depicts wetland areas which the MassGIS and TIGER data layers do not. However, this layer does not show the Nantucket Harbor. The orange box in Figure 4 depicts the hydrography data layer from the Town of Nantucket. This polygon indicates “wetland”. Note that it appears that there is additional open space showing in the orthophoto around the wetland polygon. It is unclear whether this additional open space is wetland area not captured by the hydrography layer in which case the hydrography layer would be inaccurate to up to 120 feet.

Figure 4. Positional accuracy of Town of Nantucket hydrography layer

For this project, TIGER hydrography data will not be used because the layer is not visible for the study region. Both MassGIS and Town of Nantucket hydrography layers can be used. If recreational opportunities are assessed with open space data included, the Town of Nantucket wetland polygons may be counted recreational space. However, due to lack of metadata for this layer, it is unclear whether the wetland polygons are accurate.

Quantitative Assessment of Physical Accuracy

Table 1 gives a quantitative assessment of the physical accuracy of each of the layers. The scale of original map used to create the layers is also given in the “Metadata” category where possible.

Table 1. Quantitative assessment of physical accuracy of the layers

Layer / Corresponding Figure / Positional Accuracy / Metadata
MassGIS (roads) / Figure 5 / Within 5 feet of orthophoto centerlines / Scale of digital imagery is 1:5,000
TIGER (roads) / Figure 6 / Up to 90 feet from orthophoto centerlines / Probably scanned from 1:100,000-scale topographic quadrangle
StreetMap USA (roads) / Figure 7 / Within 5 feet of orthophoto centerlines / Scale of digital imagery is less than or equal to 1:50,000
TIGER (hydro) / n/a / n/a / n/a
MassGIS (hydro) / n/a / Up to 45 feet from orthophoto shoreline / Scale of digital imagery is 1:25,000
MassGIS (open space) / Figure 8 / Up to 100 feet from orthophoto open space areas / Scale not given
Town of Nantucket (playgrounds) / n/a / Playground actual length = 185 feet, playground polygon length = 250 feet / Scale of digital imagery is 1:25,000
Town of Nantucket (sports facilities) / Figure 9 / Within 5 feet of orthophoto sports facilities areas / Scale not given
Town of Nantucket (sidewalks) / Figure 10 / Within 5 feet of orthophoto sidewalks except for some blocks which are entirely covered by the sidewalk polygons / Scale not given

As already mentioned, the MassGIS road layer (depicted in Figure 5) is fairly accurate.

Figure 5. MassGIS road layer

The TIGER road layer (in Figure 6) is extremely inaccurate.

Figure 6. TIGER road layer

The StreetMap USA road layer (Figure 7) is fairly accurate.

Figure 7. StreetMap USA

The TIGER hydrography layer did not show up in this area although wetlands as well as the Nantucket Harbor bay should be depicted as water bodies.

The MassGIS hydrography layer (Figure 3) depicted the Nantucket Harbor bay but not the wetlands.

The MassGIS open space layer (Figure 8) was extremely inaccurate.

Figure 8. Open space layer

The Town of Nantucket playground (Figure 12) layer grossly overestimated the size of the playground in the study area. See Table 1 for exact numbers.

The Town of Nantucket sports facilities layer (Figure 9) was fairly accurate.

Figure 9. Sports facilities layer

The Town of Nantucket sidewalks layer (Figure 10) is fairly accurately aligned to the orthophoto sidewalks except that some of the smaller city blocks were entirely covered by the sidewalk layer.

Figure 10. Sidewalks layer

Qualitative Assessment of Physical Accuracy

Table 2 depicts a qualitative assessment of physical accuracy of the optional layers. The best layer was the sports facilities layer. The open space layer (Figure 11) runs through all of the roads as well as sidewalks and buildings. The playground layer (Figure 12) runs through the TIGER roads, sidewalks and some parking area. The sports facilities layer (Figure 13) runs through the TIGER roads only because the TIGER road lines are not where they should be. The sidewalks layer (Figure 14) turns some city blocks in to sidewalk polygons.

Table 2. Qualitative assessment of optional layers

Optional Layer / Figure / MassGIS (roads) / TIGER (roads) / StreetMap USA (roads) / TIGER hydrography / MassGIS hydrography
(red lines) / (yellow lines) / (green lines) / n/a / Blue polygon
MassGIS (open space) / Figure 11 / Runs through / Runs through / Runs through / n/a / Runs through beach to meet the edge of this polygon
Town of Nantucket (playgrounds) / Figure 12 / No conflict / Runs through / No conflict / n/a / No conflict
Town of Nantucket (sports facilities) / Figure 13 / No conflict / Runs through / No conflict / n/a / No conflict
Town of Nantucket Sidewalks / Figure 14 / No conflict / Runs through / No conflict / n/a / No conflict

Figure 11. Open space layer

Figure 12. Playground layer

Figure 13. Sports facilities layer

Figure 14. Sidewalk layer

Appropriateness of Optional Layers

The optional layers are somewhat appropriate in terms of physical accuracy. However, the only layer which could be used for making an official map of recreational opportunities for pedestrians is the sports facilities layer. The sidewalk layer is accurate to a point, but it would have to be modified so that it would not show entire blocks as sidewalk polygons. The open space layer should not be used for publishing purposes or precise data analysis. The play ground layer should not be use as the playground polygon is almost twice the size of the playground in the orthophoto.

Completeness of Optional Layers

Completeness of required layers was already discussed in the Road and Hydrography Layer sections. The open space layer is fairly complete except for some areas which look as though they should be covered by open space polygons but are not (Figure 15).

Figure 15. Open space layer completeness

The playground layer (Figure 12) for this area is complete. The sports facilities layer is somewhat complete although American Fact Finder lists a yoga studio in the area that is not included in the sports facilities layer. The geocoded yoga studio is positioned accurately over the orthophoto (Figure 16).

Figure 16. Sports facilities layer completeness

The sidewalks layer appears to be complete for the study area.

Currency

Table 3 gives the publishing date for each of the layers. This information was provided in ArcCatalog or was obtained from each data set’s web site. These data sets are all at least 4 years old, so it is difficult to say whether or not they are complete. A tour of the study area would be the only way to know for certain whether or not the data are complete.

Table 3. Publishing dates for the data layers

Layer / MassGIS (roads) / TIGER (roads) / StreetMap USA (roads) / TIGER (hydro) / MassGIS (hydro) / MassGIS (open space) / Town of Nantucket (playgrounds) / Town of Nantucket (sports facilities) / Town of Nantucket (sidewalks)
Currency / Dec-07 / Created for 2000 Census / 2002 / Created for 2000 Census / Feb-05 / Jul-08 / Apr-03 / Apr-03 / May-04

Attribute Accuracy

The MassGIS roads attribute table includes names for some of the roads and gives the names of the bounding roads. The TIGER roads attribute table gives street names and address ranges. The StreetMap USA roads attribute table gives old and new names for roads as well as surface type (i.e., paved or not). The TIGER water body attribute table gives the size and length of water body polygons. The MassGIS water body attribute table gives the area of the water body polygon. The open space attribute table gives the site name, the fee owner, the manager, indicates whether public access is available, etc. The playground attribute table gives playground area. The sports facilities attribute table gives area of sports facilities as well as the type of facility. The sidewalk attribute table gives the length of sidewalk segments.