Data Acquisition in Central Park

Jacqueline Kennedy Onassis Reservoir

Chisa Nishii
American River College

Geography 26: Data Acquisition in GIS

Spring 2002

ABSTRACT

The purpose of this study was data acquisition on Cantral Park and to compare various kinds of maps of the area of interest. Different data sources that were acquired in this project include GIS data, drawings and DOQs from the Internet. An overlay analysis on the map was conducted and the quality of both GPS and DOQ data was discussed.

INTRODUCTION

My goal was to conduct an "active" and "passive" data acquisition and compare the data quality from both sources. Fist, I physically visited the study area and collected GPS data on my own ("active" data acquisition). Second, I searched in the Internet and acquired different kinds of maps on the central park, including drawings, aerial photographs and DOQs. ("passive" data acquisition) Finally, maps collected from the Internet were overlaid with my GPS points using computer softwares such as ArcView 3.2 and Street Atlas to investigate the quality of the data I have collected. Possible advantages and disadvantages of using GPS and DOQ over one another were discussed.

BACKGROUND

Jacqueline Kennedy Onassis Reservoir
The Jacqueline Kennedy Onassis Reservoir is located inside the Central Park, New York. The reservoir was completed in 1862 while the park was still in construction. The 106-acre water which holds over a billion gallons of water was used to distribute fresh water to Manhattan residents but it is no longer in use in that particular way. Instead, it supports varied bird species. In fact, Bird watchers have sighted five different species of gulls and over 20 species of waterfowl, grebes, cormorants and loons. The reservoir also contributes significantly to the environmental pleasure of the "run", especially in summer when water evaporates from the surface and cools the surrounding air. The reservoir is probably best known for the 1.58 mile jogging path all the way around. This path surrounding the fenced reservoir provides hundreds of thousands of joggers each year.

GPS Basics
GPS stands for Global Positioning System. It is a worldwide radio-navigation system that uses a constellation of 24 satellites to provide worldwide coverage for location information. With GPS we can tell we you are to within less than 1 centimeter, depending on the type of system used. The following are the five basics of how GPS works.
1) GPS is based on Trilateration

2) To trilaterate, GPS measures distance from the satellite.

3) To measure distance, we need good clock and at least 4 satellites.

4) Once the distance is known, we need to know the satellite position (from satellites).

5) Location is calculated.

DOQ Basics
DOQ stands for Digital Orthophoto Quadrangle. A DOQ is a computer-generated image of an aerial photograph in which displacements caused by camera orientation and terrain have been removed. Unlike conventional perspective aerial photographs that contain image distortion due to tilting of the camera and terrain relief (topography), orthophotos are uniform-scale images so that ground features are displayed in their true ground position. This allows for the direct measurement of distance, areas, angles and positions. These products combine the image characteristics of a photograph with the geometric qualities of a map and can be used in numerous GIS applications in combination with other digital data such as Digital Line Graphs or Digital Raster Graphics.

METHODS

The following describes the procedures of my project.

Acquiring GPS data in the Field

I visited the Central Park in mid March and walked around the reservoir and recorded GPS points using GARMIN 12XL. 100-foot steps were taken between each recorded points and total of 37 GPS points were acquired. 30 seconds were measured in each stop to record the average distance from the satellite. To assure data accuracy, number of satellites visible on the GARMIN screen was recorded at each stop as well. The GPS data was then exported and plotted in the WAYPOINT ++ .

Acquiring DOQs from the Internet

A random search was conducted over the Internet. Common search engines such as Yahoo, Alta Vista and Google were visited for the purpose of finding DOQs from the Internet. Words entered in the search sections were those such as “Central Parks and DOQs”, “GIS in Central Park, NY.” More than a hundred websites were observed in search of maps including DOQs that depict the reservoir. One DOQ was purchased over the Internet under the Instructor’s authorization.

Comparing GPS data and DOQs

First, the field collected GPS data was downloaded in the software WAYPOINT++ to be observed. This information was first transferred to the software, USA Street Atlas, to conduct the first overlay with an existing image of a reservoir in the software. The GPS information was then transferred into ArcView 3.2 by saving the GPS information as a dbf file. The DOQ that was purchased from the Internet was also exported to ArcView 3.2 to conduct an overlay analysis. There were no other DOQs or images that were successfully transferred to ArcView 3.2.

RESULTS

The GPS data was collected successfully.

Figure 1. demonstrates all the 37-GPS points that I acquired around the reservoir. The data was originally collected in UTM NAD 27.

Fig 1. Waypoints Around the Reservoir

Figure 2. is a collection of the varied maps of the reservoir that were acquired from the Internet.
Fig 2. Various maps of the Reservoir

The first overlay analysis was conducted in USA Street Atlas. Figure 3. shows the GPS points that were directly downloaded to the USA Street Atlas.
Fig 3. First Overlay in USA Street Atlas

The second overlay analysis was conducted in ArcView 3.2.
The projection of the original GPS data (UTM NAD27)was transformed to UTM NAD83 to adjust it's projection to the DOQ that was used as a base map. Figure 4. shows how the GPS points overlaid with the DOQ.

Fig 4. Second Overlay in ArcView

DISCUSSION

The GPS points on the first overlay are very far away from the reservoir boundary depicted on the picture in the Street Atlas. On the other hand, GPS points on the second overlay are very close to the reservoir boundary that is shown on the DOQ image. Sources of error for the GPS data may include atmospheric interference and therefore delayed and insufficient spatial information received from the satellites. However, GPS and DOQs are probably the most reliable and accurate data sources among all others. It’s probably now a challenging debate to compare the effectiveness and efficiency between GPS and DOQ data because they both have advantages and disadvantages. Possible advantages of using GPS data are infinite resolution, exceeds national map accuracy standard and low in cost. However, GPS may also have disadvantages including missed information due to equipment failure and its slow and labor-intensive nature. DOQs, on the other hand, can depict spatial features of large areas very quickly and meets the national map accuracy standards. However, some disadvantages are that they are extremely costly and small and detailed spatial features may not be detected on the image by human eyes.
CONCLUSION

My original goal was to conduct various overlay analysis to perform a thorough comparison between the accuracy and effectiveness of different data sources including GPS data and DOQs. However, the overlay analysis and the cross-examination of the data quality between different data sources were only limited to two in this project due to various reasons. First, it was very difficult to acquire projected data that was easily exported to ArcView to conduct overlay analysis. Data acquisition over the Internet certainly required a lot of time, effort and tremendous amount of luck. The other reason for the insufficient overlay analysis was my limited knowledge on projection. I could have performed more overlays if I could project unprojected JPEG images that were available over the Internet. Understanding more about projections and the software itself would be helpful to proceed and expand my studies in the future.