Emily McRae's GIS Blog

This weeks module uses an example of GPS points and a reference point to demonstrate methods of determining accuracy and precision in GIS.  The above map uses a set of given GPS point locations to determine the average location of all points, and then the difference from actual points to this average. The 68th percentile is determined, and the distance away from the average location at the 68th percentile is used to create a buffer.  Calculating Precision Precision refers to the closeness in value a set of values, in this case geographic coordinates, are to each other. It does not indicate anything about the closeness to the actual value, or the actual location of the point in this case.  In this example, a GPS is used to take 50 sample measurements of a single location. The precision of these values can be measured using percentiles. It is generally accepted that those values in the 68th percentile, or those values which make up 68% of all values when ordered numerically from smallest

 This module was on conducting damage assessment on coastal flooding using ArcGIS Pro analysis tools. A case study of Hurricane Sandy is used. Using imagery and various analytic tools, a general assessment of structure damage caused by flooding and wind is conducted for an area in New Jersey.  Structures by structural damage level after Hurricane Sandy. Process summary: 1) 2 mosaic datasets are created, one for the study are before the storm and one for after, using aerial rasters. The mosaics are created within the gdb being used. The result are two projected aerial images with grids highlighting the study area. When turning on and off these two layers, damage such as flooding and missing buildings is apparent. 2) Within the gdb, new domains are created that will be used to categorize damage by type and severity. This is done by adding Domains under the gdb, and the categories for Inundation, Structure Damage, Wind Damage, and Structure Type are added alon with options for each domain

In this module, various methods of assessing coastal flooding are used to determine flood damage. In this first example, the affects of Hurricane Sandy on a part of the New Jersey coastline is assessed using DEMs. A DEM of the area before the storm and a DEM of the area after the storm are used, along with satellite data from before the storm, the assess which areas experienced erosion and deposition, likely from debris. The study are DEM of after the storm is subtracted from the pre-storm DEM to generate a change layer, which can be used to assess areas based on their surface elevation change. This method, combined with visual analysis and comparison of previous buildings and surface features, can be used to determine where buildings have been damaged or washed away. In this second example, storm surge of 1 meter is analysed as it is likely to affect building locations in Collier County, Fl. Two data sources are used, a Lidar DEM and a photogrammetry DEM from the USGS. The L