|by Neil Moomey for University of Alaska Anchorage, GIS & the Marine Environment (GIS 333), Dr. Thom Eley. Fall 2003.|
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The objective of this project was to combine elements of the most useful maps, charts, and aerial photos of Alaska's Green Island in Prince William Sound to create the most data rich map possible. Such a map would be comprehensive enough to use as a land and sea navigational aid, analysis of terrain for hiking, hunting, and photography, and the bays for fishing, diving, kayaking, mooring, and camping locations. It could also be used for scientific research. Currently several maps are required to accomplish all of these tasks and none of the published maps show as much detail or accuracy as the aerial photos purchased at AeroMap. The topographic map in particular is very out of date missing lakes, steams and vegatation. Parts of the nautical chart have been updated recently however much of the coastline is still based on pre 1964 earthquake data. As I worked on this project and compared data sources I discovered which types of maps were the most useful and accurate for this region. I also had to strike a balance between what data I wanted to use and the reality of what I could produce within the scope and resources of this project. Since maximizing the information on this map was the primary goal I choose to use aerial photography complimented with nautical chart data. Printing a poster sized map and laminating would produce a weatherproof map which could easily be folded and carried in a backpack for hiking or used for kayaking and boating.
Materials and Methods:
To be useful this map needs to be as detailed as possible. Four aerial photos from AeroMap in Anchorage, AK were scanned at 600dpi to create a map 30 by 30 inches printed at 300dpi. Using ArcGIS 8.3 the images were georeferenced to a DRG topographic map in UTM Zone 6, NAD27 projection. A NOAA Electronic Navigational Chart (ENC) was converted from S-57 to ESRI Shapefile format. Two converters were necessary for this. NOAA produces an ENC Data Handler extension for ArcView 3.x however the shapefiles would not line up correctly with the NAD27 DRGs. SAFE also creates a converter however the depth field in the Sounding layer was missing. I was able to use the SAFE created shapefiles by replacing the Sounding dbf file with the one created from NOAA's ENC Data Handler. To lay the vector data over the aerial photos all of the polygon layers were made transparent fill. Nautical Chart symbols for point features and graduated values for depth contours were also used. I then converted the soundings in meters to fathoms and added sounding labels. The map was then exported to a JPEG image at the maximum dpi. I experimented with blending, removing, and making transparent the off shore region of the raster image. In the end I decided there was just too much useful information to give up. It would have been ideal to blend the water into the background but from a technical stand point I could not do this to my liking.
I could not find a nautical chart compass rose or a nautical chart lat/lon neatline so I wrote Java programs to create these graphics and save as a transparent raster image. I then used Paint Shop Pro to put finishing touches on both drawings and paste into the map. PhotoShop and Paint Shop Pro were used to assemble the map and add text. The final image was saved as a Portable Network Graphics (.png) file to retain the highest image quality and the smallest possible file size. PNG images have the same quality as TIFF with smaller file size.
There were many unexpected challenges with this project. Working with such large files used allot of computer resources. A PC with a 1.47GHz processor and 512MB of RAM was marginally adequate. Often I had flatten the layers to conserve memory. A better ENC to Shapefile converter and more nautical chart tools such as a compass rose and lat/lon neatline would have greatly simplified this project. One of the challenges with using multiple data sources is alignment of layers. Errors in data created some misaligned structures. Only an official printed version of the NOAA nautical chart is recommended for boat navigation. In addition no nautical charts or topographic maps have been completely updated since the 1964 earthquake which raised the land on Green Island up to ten feet. The resulting map was as good or better than I expected with the data available. The detail of the composite aerial photo is very impressive and most of the nautical chart data is accurate. For liability reasons I cannot officially recommend this map or any of it's data as suitable for navigational purposes. It is merely an aid to be used with other navigational tools.
By it's nature aerial photography does not show bathymetric or elevation data. Vector overlays are an excellent way to display this missing data. Since the topographic data was so poor and the island was relatively flat I decided to omit the elevation data except for the three hilltops. Hybrid maps are becoming more popular as electronic data and GIS tools become more abundant. Combining topographic maps or aerial photos with vector data for land use is nothing new however the combination of aerial photography with nautical chart vector data is less common. I was unable to find any examples forcing me to experiment with several ideas before creating something which met my project goals. I originally hoped to update the existing topographic map however it was so inaccurate I would have had to create one from scratch which was outside the scope of this project.
The use of electronic mapping will continue to grow at a rapid rate. Breakthroughs in remote sensing, the increased availability of electronic data, better software, faster more powerful computers, and improved portable chart plotting GPS and mapping devices makes electronic mapping more feasible. The recent trend in 3-D plotting will also bring unique opportunities. Traditional paper charts and maps will play a less important role as this trend continues. As software tools for ENCs are invented and ENCs are improved it's feasible that official paper charts may be constructed from ENCs.
I learned many lessons from this project. It was the first time I had used ArcGIS 8.3, large aerial photos, or NOAA's Electronic Navigational Charts. Working with multiple large files at once was a challenge. More RAM and CPU power would have helped. Aligning layers created from different data sources and different methods is also challenging. Changes in the land structure from erosion and earthquakes, human error, and the accuracy of remote sensing all affect the data. In hindsight I think it would have been easier to find or create a NAD83 topographic map for georeferencing the images. The ENCs from the ENC Data Handler are projected to UTM NAD83.
The rapidly advancing GIS and Marine GIS technology is very exciting. New breakthroughs will pave the way to more useful charts and navigational aids in ways not thought of yet. Combining maps and data of various formats together can be a powerful tool for analysis. Overall I am pleased with the final product and I know it will be useful to me and others.