Accessibility and Cost Surfaces in the Boundary Waters Canoe Area, Minnesota

Transcription

1 Accessibility and Cost Surfaces in the Boundary Waters Canoe Area, Minnesota Caroline Rose, Chandler Sterling, Chase Christopherson, Matthew Smith Geography 578 May 13, 2011

2 Table of Contents Capstone 2 Introduction. 2 Conceptualization 3 Implementation 5 Results.. 8 Discussion.. 9 Conclusion References Figures 13 Appendix A: Metadata 18 Appendix B: Programming. 22 1

3 Capstone Statement Analyze the accessibility for canoeing in the Boundary Waters Canoe Area Wilderness. This assessment could aid canoe trip leaders in planning for travel and emergency evacuation. The Boundary Waters Canoe Area Introduction (Figure 1) The Boundary Waters Canoe Area Wilderness (BWCAW) is located in northeastern Minnesota on the Canadian border (Figure 1). The BWCAW is nearly 1.3 million acres in size with over 1,000 lakes and streams, over 1,500 miles of navigable canoe routes, and over 2,000 designated campsites for canoeists. To traverse the BWCAW one must navigate its elaborate network of lakes, streams, and portages. Dan Pauly, an expert on the BWCAW, defines portaging as an overland trail connecting two bodies of water (56). To get from one lake to another one must cross a portage, carrying all gear and canoes across the length of the trail. All portages are not equal; they vary in length, width, slope, maintenance regime, and trail and 2

4 landing condition. The accessibility of areas within the BWCAW depends on the distance from the entry point and difficulty of each traversable portage. Why Accessibility within the BWCAW? Every year thousands of individuals set off to explore remote areas within our country s National Parks and Wilderness systems. The BWCAW sees over 250,000 people annually navigate through more than one million acres of remote wilderness, making it the most used area within the National Wilderness Preservation System (NWPS). The high volume of visitors contains people with various degrees of wilderness experience, and it is important that people choose routes and trips that appropriately reflect their ability. Through this project we visualized relative accessibility by canoe throughout the BWCAW from entry points to remote locations. Conceptualization The goal of this project was to visualize accessibility from entry points throughout the BWCAW. The key concept employed in our project is accessibility from entry points. The project utilized 59 entry points, where non-motorized craft can officially enter the BWCAW. The extent of the project was the network of streams, lakes, and portages accessed by canoe and on foot. A portage is an over-land trail that connects two bodies of water; in the BWCAW, portages connect to both lakes and streams. We used a cost distance analysis to derive accessibility. A cost distance analysis uses the cost or impedance to traverse each cell as a distance unit (Chang, 234). We defined high accessibility as a low cost distance, where cost distance is a total cost accumulated, beginning at a source point, through travel over bodies of water (lakes and streams) and portages. 3

5 Variables The variables used to analyze accessibility were cost and distance. Cost values were assigned to each cell in a grid, representing the difficulty of traversing that cell. A cost distance surface was created by examination of the network of most efficient possible routes within the BWCAW beginning at entry points, traversing lakes and portages, and avoiding barriers to travel. Utilizing the variables cost [difficulty] and distance, we were able to visualize accessibility from entry points to all locations within the BWCAW. Operationalized Variables A cost distance analysis was used to determine accessibility across the Boundary Waters Canoe Area Wilderness (BWCAW) based on the least cost path from an entry point. The cost distance analysis took into account a cost surface, consisting of the cost of paddled surfaces, which are crossed by canoe, as well as portaged surfaces, which must be crossed on foot. All grid cells (lakes, streams, and portages) in the analysis received a base value of 1. Open water, which contained both streams and lakes, was considered the least difficult to traverse and was given no additional cost. Portaging was more costly than paddling as it included the unloading and loading of gear, transport of all gear by foot, and even multiple trips over the portage trail. For this reason, all portaging areas were assigned a greater difficulty than paddling areas. To reflect the greater difficulty of portaging, many portages were assigned a difficulty rating based on expert opinion; this system was extended to all portages based on length. Portage ratings had a range of This rating was divided by portage length in grid cells to assign each grid cell within a portage an equal share of the overall difficulty for that portage. Certain areas, including 4

6 shallow streams, rapids and waterfalls, posed significant barriers to travel and were assigned NoData values, excluding them from the network of travel surfaces. For the purposes of this analysis, land without portages could not be crossed and all water within the lakes and streams layers was considered paddleable. All land that was not considered a portage was assigned a No- Data value. These values were combined into a cost surface layer to guide the GIS in identifying the cost distance from the entry points to any part of the network. For a diagram of the conceptualization, see figure 2. Implementation Data Preparation The general implementation of our analysis followed figure 3. The layers used included the BWCA boundary, lakes, entry points, portages and barriers. The final output was a combination of all layers into a cost surface raster. The data layers had to be properly prepared before they were ready for the cost surface raster operation. The procedure to prepare the lakes, streams, and portages layers was repetitive. We first set the spatial reference then used the BWCAW boundary layer to clip the layers to the study area. The three layers were first clipped within the study area. Some entry points, however, existed on lakes outside of the BWCAW. The layers were then selected by location, selecting for all lakes, streams, and portages that intersect with the BWCAW boundary, to extend the study area to include the entirety of lakes partially inside and partially outside of the BWCAW boundary. After selecting for all attributes within the boundary itself, we then applied topology rules to the streams layer because many of the stream polylines ran through the lake polygons. We 5

7 erased any section of a stream that intersected a lake. Next, we rasterized the stream and lake layers to a grid cell size of 10 meters. After rasterization, we reclassified each grid cell in both the stream and lake layers to a value of 1. Reclassification was the final step in data preparation for these two layers before adding them to the cost surface. After we prepared the lakes and streams layers, we began preparation of the portage layer. We obtained the portage layer in.kml format and converted it into a shapefile using OGR2OGR (see Appendix B). Once in shapefile format, we investigated the file and noticed that the polylines of the portages did not precisely match up to the lake polygons. To remedy this issue, we used spatial adjustment (edge snapping) to match the portage polylines to the lake polygons. After snapping, errors still occurred in the portage layer because some of the polylines were within the lake polygons. They were identified using topology rules and manually adjusted to remove portage polylines that were within the lake polygons. Once we fixed the portage polylines and each portage was in its correct location, we had to assign a cost rating to each portage for addition to the cost surface raster layer. We created a list of each portage s difficulty, the lakes that the portage went to and from, and the length of the portage. We determined the difficulty of each portage based on the book Exploring the Boundary Waters by Daniel Pauly. He lists many portages within the BWCAW and gives each portage a difficulty rating from one to ten based on characteristics such as portage length, slope, muddiness, rockiness, and landing quality. We built the table of portage difficulties using these ratings and then joined the table to the existing portage attribute table. Because not all portages within our shapefile were included in the constructed portage table, difficulties of the unmatched portages were generated based on length class. We determined length class by grouping all portage lengths into 10 classes using the natural breaks classification scheme. We generated the 6

8 average difficulty of known portages in each class and then applied that value to the unmatched portages. Once each portage had its own difficulty rating, we created two rasters of the portage layer. One raster layer was rasterized by difficulty rating and the other was rasterized by shape length. We then performed raster calculation to derive the final value for each grid cell within the portage layer. The raster based on difficulty rating was divided by the raster based on length to give each grid cell its appropriate cost rating, distributing the difficulty of each portage among all of its grid cells. With this final step, the portage layer was ready to be added to the cost surface raster. Within the BWCAW there are waterfalls, rapids, and other geographic features which are unsafe or impassible for canoeing. We did not have a shapefile of the locations of the barriers, so we investigated the portage layer for suspected barrier locations. For each portage that appeared to avoid a stream, a square polygon was placed there as a barrier. Barriers were rasterized and assigned a NoData value, effectively deleting a section of water, to force the cost raster to seek a different route (i.e., to take the nearest portage instead of paddling). The barriers layer was created to model the travel surface more realistically under the assumption that portages are created to bypass some hazard or barrier to paddling. The entry points layer was also obtained in.kml format. We used OGR2OGR to convert it to a shapefile (see Appendix B). We discovered that some of the locations of the entry points were not touching the lakes or streams, which resulted in an erroneous cost surface raster excluding those points. By moving the entry points to be completely surrounded by water, then rasterizing the entry points, we successfully generated an acceptable layer of source points to be used as points of origin in the cost distance analysis. 7

9 Cost Raster Paddling surfaces, consisting of the lakes and streams, were considered the easiest to travel. Each paddling surface grid cell was given a base travel cost of 1. Portages were modeled as more difficult than paddling and received the base cost of 1 plus an additional cost based on rating. The portage ratings from Exploring the Boundary Waters, ranging from 1 to 10, were multiplied by ten, producing ratings of The new rating for each portage was divided by the total number of grid cells in that portage to find a cost value for each grid cell in each portage (See Figure 4). The length of the portage in grid cells was found by dividing the length of the portage by 10 meters, the grid cell size. The barriers layer was created over streams with known or suspected impedance to travel such as shallow water, rapids, or waterfalls. We wanted to include only travelable areas. Barriers were given a rating of No-Data to make them impassable and force the analysis to take portages and avoid non-navigable streams. Land cover that was not distinguished as a portage was given a value of No-Data. Portaging over land that covered by vegetation is very difficult; we considered this impassable and out of the scope of our project. All layers were added together using raster calculation to make a cost surface at a spatial resolution of 10 meters. Cost distance analysis was used to create the final output map. This operation calculated the least accumulative cost distance from each cell to the nearest entry point. Results Our output is a static map of cost distance throughout the study area (see fig. 5). The low value is green in color. This indicates relatively easy access. The high value is red in color and indicates relatively difficult access. The numeric values of low and high are not important; the project can only meaningfully show relative cost distance and accessibility. Cost distance values 8

10 were produced by accumulation of travel cost over each grid cell on the cost surface. The inset depicts a good example of accessibility within the network of lakes, stream, and portages. The area closest to the entry point in green as it is close and there are not many portages to cross. The output becomes green to yellow to orange moving further from the entry point. There are, however, streams that are close to the entry point and yet have a red color. This is because there are no portages connecting to these streams. The intervening land has a No Data value. To get to these areas one must travel through the network of streams, lakes and portages, accumulating travel cost over more grid cells and making it relatively more difficult to get to. The same analysis was run after increasing the portage ratings per grid cell by an order of magnitude from the original design (see fig. 6). The output produced a similar map to our initial map, highlighting the same general areas of relatively harder accessibility. When portages were reassigned to just the base value of 1, the output produced a near identical map as our original design (see fig. 7). Discussion Assumptions We must acknowledge that our model is a drastic simplification of the world. Some assumptions were inherent in our model of the study area as a raster grid: the grid takes a complex and detailed physical world and simplifies it to 10 meter blocks of homogeneous area belonging to one of a finite number of classes. The canoeist s true travel surface of marshes and shallows and of rocky, muddy, slick or steep ground has been reduced to only squares of water and portage in a vast landscape of NoData values. The model assumes that all water cells are equally easy to traverse, that each cell in a particular portage is equally difficult, and that each portage s difficulty can be reduced to a static, general rating. While many portages were given a 9

11 rating from reference material, ratings for many unlisted portages were assumed to be the average for that portage length. This led to another major assumption: that the length of each digital portage line in our model was representative of true portage length on the ground. We recognized that these lengths were very approximate, but we had no better alternative; no existing map accounts for every twist and turn of a portage trail. Water cells were assumed to be passable except where a portage provides an alternative. Many of these areas may be too shallow to travel either seasonally or in general. The barriers layer was based on the assumption that all portages exist for a good reason. A barrier was positioned to block any paddling route that would bypass a portage. While some barriers to paddling -- rapids, waterfalls and shallow, rocky streams-- are known through a group member s field experience, most were simply suspected to exist. Variable conditions were impossible to capture in our assessment. The wilderness changes hour to hour, month to month, and year to year. Forest fires come and go, blocking access while they burn and leaving a changed landscape behind. Wet and dry years mean variable water levels. Inclusion of these conditions, however, would make the analysis too casespecific. We had to assume fair weather conditions, having no way to account for three foot waves and headwinds blasting across the vast expanse of Saganaga. High winds disproportionately affect large bodies of water, where waves build up, while small or narrow areas may be sheltered; this might affect the geographic distribution of cost distance. Although difficult to measure and constantly changing, variable conditions have a great impact on wilderness travel; their exclusion is a limitation of our project. Our approach is limited; the travel surface is simplified, portage lengths (which affect weighting scheme) are very approximate, we assume the presence of barriers, and the model 10

12 offers no way to account for variable conditions. We recognize these limitations, but feel that we have done our best with the available resources. Subjectivity The greatest subjective decision in the development of our project was the cost of portage travel relative to paddling. As our results showed, this relative weighting makes little difference in the overall pattern of accessibility across the Boundary Waters. Other decisions were reinforced by our reference material. This analysis relied heavily on Daniel Pauly s book Exploring the Boundary Waters for subjective information regarding the study area. The publication supported several important decisions, including: - The exclusion of land without portages, which could, in theory, be accessed by bushwhacking; It is all but impossible to bushwhack significant distances through the dense forests of the Boundary Waters, particularly with a canoe and gear (60). - The travel cost of portage surfaces; Pauly gives many BWCA portages a difficulty ranking from 1 to 10, based on personal experience (56). - The assignment of average rating by length for unlisted portages; Some portages do not have rankings, typically because they are of average difficulty for their length (56). While we recognize that these choices might be made differently and may affect the outcome of the project, we are confident that we have made informed decisions. 11

13 Uncertainty Every layer in the project included some level of uncertainty. Portage length is uncertain as is the exact location of each portage s connection to a lake or stream. Portage ratings are uncertain as difficulty may vary with season, and portaging cost relative to paddling depends on the gear, efficiency, organization, and abilities of the specific group. The layer of water, including lakes and streams, is accurate enough for our purposes in terms of location, but its classification as paddleable is uncertain. The precise locations and often the existence of barriers are uncertain. It would take an extensive survey to quantify these uncertainties. Although they limit our analysis, we consider these ambiguities an innate characteristic of our study area. It is the nature of wilderness to be wild and uncertain; part of the adventure of wilderness travel is to face the unknown. Conclusion The goal of this project was to visualize accessibility from entry points to all areas throughout BWCAW. The map output was able to demonstrate areas of harder accessibility relative to areas of easier accessibility. Portage difficulty did not weigh into the analysis as originally predicted; it was the distance from entry points that largely drove the cost distance analysis. The output map will help in planning canoe trips for groups of all skills and abilities. Beginners will want to stay in areas of easy accessibility that are closer to entry points. This will hopefully make their first experiences in the Boundary Waters more enjoyable. It will also allow for a faster exit in case of an emergency situation. The more experienced paddlers who seek a challenge may want to push themselves to explore areas that are less accessible and relatively more difficult to get to. 12

14 References Chang, Kang-tsung. Introduction to Geographic Information Systems. New York, NY: McGraw-Hill Higher Education Pauly, Daniel. Exploring the Boundary Waters. Minneapolis, MN: University of Minnesota Press United States Forest Service (USFS). The BWCAW Act. Special Places, Superior National Forest P0os3gjAwhwtDDw9_AI8zPyhQoYAOUjMeXDfODy-HWHg-zDrx8kb4ADOBro- 3nk56bqF- RGGGSZOCoCAPi8eX8!/dl3/d3/L2dJQSEvUUt3QS9ZQnZ3LzZfMjAwMDAwMDBB ODBPSEhWTjJNMDAwMDAwMDA!/?navtype=BROWSEBYSUBJECT&cid=stelprd b &navid= &pnavid=null&ss=110909&position=not%20yet% 20Determined.Html&ttype=detail&pname=Superior%20National%20Forest- %20Special%20Places (accessed 4 March 2011). Figures Figure 1 (embedded in text): 13

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18 Figure 5: Figure 6: 17

19 Figure 7: Appendix A: Metadata Identification Information: Citation: Citation information: Originators: University of Wisconsin Madison, Department of Geography, Geog 578 *Title: barriers_new *File or table name: barriers_new Publication date: May 13, 2011 *Geospatial data presentation form: vector digital data Description: Abstract: Known and suspected impedance on water travel. Barriers were placed over streams where there are impassable obstacles such as rapids, shallow water, waterfalls, etc. On streams that ran parallel to portages, but did not have known barriers, it was assumed the portage was there for a reason and a barrier was created over the stream to force the cost distance analysis over the portage layer. Purpose: Added to cost surface layer. Excludes cost distance analysis over impassable streams. Calendar date: unknown Currentness reference: publication date Status: Progress: Complete 18

20 Maintenance and update frequency: None planned Spatial domain: Bounding coordinates: *West bounding coordinate: *East bounding coordinate: *North bounding coordinate: *South bounding coordinate: Local bounding coordinates: *Left bounding coordinate: *Right bounding coordinate: *Top bounding coordinate: *Bottom bounding coordinate: Theme keywords: Barrier Theme keyword thesaurus: Impediment Access constraints: None Use constraints: None Contact organization: University of Wisconsin Madison, Department of Geography, Geog 578 *Native dataset format: File Geodatabase Feature Class *Native data set environment: Microsoft Windows XP Version 5.1 (Build 2600) Service Pack 3; ESRI ArcCatalog Data Quality Information: Process description: Created polygon layer over stream layer. Process date: Process time: : Process description: Dataset copied. Process date: Process time: Source used citation abbreviation: E:\Geog578\barriers\barriers_2 Process description: Dataset copied. Process date:

21 Process time: Spatial Reference Information: Horizontal coordinate system definition: Coordinate system name: *Projected coordinate system name: NAD_1983_UTM_Zone_15N *Geographic coordinate system name: GCS_North_American_1983 Entity and Attribute Information: Detailed description: *Name: barriers_new Entity type: *Entity type label: barriers_new *Entity type type: Feature Class *Entity type count: 857 Attribute: *Attribute label: OBJECTID_1 *Attribute alias: OBJECTID_1 *Attribute definition: Internal feature number. *Attribute definition source: ESRI *Attribute type: OID *Attribute width: 4 *Attribute precision: 0 *Attribute scale: 0 Attribute domain values: *Unrepresentable domain: Sequential unique whole numbers that are automatically generated. Attribute: *Attribute label: OBJECTID *Attribute alias: OBJECTID *Attribute definition: Internal feature number. *Attribute definition source: ESRI *Attribute type: Integer *Attribute width: 4 *Attribute precision: 0 *Attribute scale: 0 Attribute domain values: *Unrepresentable domain: 20

22 Sequential unique whole numbers that are automatically generated. Attribute: *Attribute label: Shape *Attribute alias: Shape *Attribute definition: Feature geometry. *Attribute definition source: ESRI *Attribute type: Geometry *Attribute width: 0 *Attribute precision: 0 *Attribute scale: 0 Attribute domain values: *Unrepresentable domain: Coordinates defining the features. Attribute: *Attribute label: Id *Attribute alias: Id *Attribute type: Integer *Attribute width: 4 *Attribute precision: 0 *Attribute scale: 0 Attribute: *Attribute label: Shape_Leng *Attribute alias: Shape_Leng *Attribute type: Double *Attribute width: 8 *Attribute precision: 0 *Attribute scale: 0 Attribute: *Attribute label: Shape_Length *Attribute alias: Shape_Length *Attribute definition: Length of feature in internal units. *Attribute definition source: ESRI *Attribute type: Double *Attribute width: 8 *Attribute precision: 0 *Attribute scale: 0 Attribute domain values: *Unrepresentable domain: Positive real numbers that are automatically generated. 21

23 Attribute: *Attribute label: Shape_Area *Attribute alias: Shape_Area *Attribute definition: Area of feature in internal units squared. *Attribute definition source: ESRI *Attribute type: Double *Attribute width: 8 *Attribute precision: 0 *Attribute scale: 0 Attribute domain values: *Unrepresentable domain: Positive real numbers that are automatically generated. Metadata Reference Information: *Metadata date: *Language of metadata: en Metadata contact: Contact information: Contact organization primary: Contact person: REQUIRED: The person responsible for the metadata information. Contact organization: University of Wisconsin Madison, Department of Geography, Geog 578 Appendix B: Programming Two layers we used to produce our final output, portages and entry points, were originally in.kml format. To convert them to shapefile format, we issued an OGR2OGR command in the FWTools shell. FWTools is an open source GIS/RS binary kit for Microsoft Windows and Linux (more information on FWTools can be found here: The command issued was ogr2ogr -f output format OutputDataSource InputDataSource where output format was ESRI Shapefile, the OutputDataSource was the name of the new file, and InputDataSource was the.kml file. 22