Posted on June 20, 2013 by James Douglas
Today was the first real day of field work where we were pursuing our individual projects. I teamed up with Julianna because our projects require some similar data collection. Our task for the day was to try to collect data to correct the existing DEM by walking up the stream corridor and taking points to characterize the elevation change. We started as close as we could get to the outlet of the main channel- we were stopped by a fence that prevented access to the stream as it crossed into private property. I feel like this might technically be a violation of the law, but I guess I don’t really know the laws regarding public access to water in Hawaii. Anyway, we slowly began working up the stream, collecting points wherever the topography visibly changed- mainly at the top of riffles. While we were at each point, we recorded various information about the environment for future reference, as well as the temperature, depth, and flow where appropriate. Since we occupied each GPS point for 175 seconds to try and improve accuracy, this was very tedious work. By the end of the work day, however, we had worked more than halfway up the channel and collected 36 points, and I felt satisfied with the amount of work we had accomplished.
The next step was to see if any of the data we had collected would be useful. After dinner, I exported the data into a shapefile so we could assess the Z-values that we had obtained. However, after laying the points over the DEM and comparing the elevation values to the GPS data, we realized that the DEM recorded its values in Mean Sea Level, while the GPS recorded values in the Height above Ellipsoid. This meant that the values were essentially in a different unit, and in order to convert one to the other, we required the Geoid Height. Although theoretically, this is a simple task of subtraction, the Geoid Height is not a value that is kept in a database like elevation data, so without some further help from the professors, we were stuck. The plan is to try and see if there is an easy way to accomplish the conversion tomorrow morning so that we can assess the accuracy of the GPS data and determine if it makes sense to continue collecting elevation data on the ground. If the error is too great, it doesn’t make sense to continue because we wont be correcting the DEM. I hope this issue can be solved expediently so that the research can continue.
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