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Lab 02 Coordinate Data, Projections & Transformations

Introduction

As part of Lab 02, students participating in WATS 6920 were given total station data from a stream habitat survey in a stretch of the Bear Valley Creek. The survey data are used to assess the habitat of salmonids in the Columbia River Basin. The data was very precise but when the points are added to ArcGIS they are not given a specified coordinate system. Completion of this lab consisted of: 1) show the unprojected points on a map using ArcGIS; 2) use the Columbia Habitat Monitoring Program (CHaMP) Transformation Tool to transform the unprojected data into projected data at the study site; 3) share the transformed data using an interactive map.

Methods

This section describes what was done with the survey data and how it was transformed to fit the study site.

Task 1

The first task was to import the unprojected data provided by the instructor and create a map that showed the data. The data was very accurate so grids were added to the map. Labels were also added to the map to help understand the different survey points.

Task 2

The second task was to transform the unprojected data using the Columbia Habitat Monitoring Program (CHaMP). This tool helps move, or translate, and rotate the data to a correct position and location.

Task 3

The third task was to create an interactive map that allows for users to explore the study site and the surrounding area. A Google map was used using a kmz file that showed the study site.

Results

Task 1

ArcGIS was used to take the data and plot them on a assumed Cartesian coordinate system. A Cartesian coordinate system is a way of plotting data using two points. In math Cartesian coordinates commonly use x and y coordinates, in maps it is more common to use Northing and Easting coordinates. Figure 1 shows the unprojected survey points on the assumed coordinate system in the map on the left side of the figure. The map shows that it is an assumed coordinate system because the values shown for the grids are not indicative of the location. While the survey is very accurate, the map of the unprojected points needed some work. The units are unknown, the direction of north is not known, and the data need to be transformed to better represent the true location of the study site. A pdf of the map can be downloaded here.

Figure 1. Unprojected and projected data in Bear Valley Creek, Idaho. 


Task 2

To transform the survey data to the correct coordinates the CHaMP Transformation Tool was used. This tool helped transform the survey data from an unprojected assumed coordinate system to a projected coordinate system. The tool also helps rotate, or orient the data, so directions are known (Wheaton). To transform the unprojected data to the right coordinates the data was moved, or translated to the correct coordinates. Then the data was rotated so the North arrow was pointing vertically. The transformation that was used to do this is called the Affine Transformation. This transformation uses Northing and Easting coordinates and a minimum of three control control points. Even with the accurate data provided by the survey, error is still present. Figure 2 shows the error associated with using the different possible transformations. The transformation that was chosen was 3/GPS 2, because this choice had the least amount of error associated with the transformation. The resulting map is shown on the right side of Figure 1.

Figure 2. CHaMP Transformation Tool hinges and errors.

Other transformations include Similarity and Projective (ESRI, 1995). Similarity transformations require at least two control points but three points are recommended. This was not chosen because the Similarity transformation requires a scale be set and this did not need to be changed. The projective transformation uses a complex formula which takes transforms data from aerial photography. This was very complex so this transformation was not used.

Task 3

The interactive map was created using Google maps and is shown below. Users can utilize this map and use the zoom and pan controls to verify that the site is in the correct location and projection.

Streambed Elevations


Conclusions

As part of the WATS 6920 course students took survey data that was very accurate, however this data needed to be transformed to correctly display the true placement and projection. Transformation is used when data is imported and no projection is associated with the data. In cases where data has been either previously transformed or taken with correct projections, ArcGIS can automatically transform and project data in their correct position. 

References

Wheaton, J. (n.d.). Champ transformation tool: An arcgis add-in. Retrieved from http://ctt.joewheaton.org/home/download-tool. 

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Barry Prettyman,
Jan 20, 2014, 11:37 AM
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