Lab 2: reclassification and updates

Introduction

This lab deals with an anlysis using simple reclassification and updates. The data used for this exercise are from a real project that I worked on; in fact it was the last project that I worked on before coming to while living in Florida.

You will find the required data set at /home/sanduku/classes/5930/lab2.apr; open up this dataset in arcview, and you should see a screen that looks thus:

The data are from Jupiter Inlet, on the Atlantic Coast of Florida. The length of the study area from left to right is about 2 miles; the Atlantic ocean is to the immediate east of the map, and the Loxahatchee wildlife refuge is to the northwest. The shoreline in this map is mainly high-priced residential shorefront property, where most of the homeowners have boats (ok, yachts) in their back yards. You will notice that there are three coverages involved:

These three layers are three different data types. Part of the purpose of this exercise is to demonstrate to what extent data of various types can be used in an analysis together.

housekeeping

To complete this exercise, save this file to your local directory. You can save it anywhere you like, but I recommend that you make a directory called "lab2" or something like that. You will see that most exercises that we do will involve about a gajillion intermediate files, and it will be useful to have scratch files for various exercises segregated into understandable directories.

The data layers

If you click on the "key" area on the left hand side of the display, you will see the three data layers mentioned.

  1. Boats: This is the result of a boat owner survey. A group of residents in the canal system were asked varioud details about themselves and their boats. Their homes are mapped, and their responses to their surveys has been attached as attributes to their locations. If you click your mouse on the key in the left hand corner (the part that looks like this: ), and then click on the "feature information button" , you will see the attributes for each boat as you click on it. These attributes are the results of a survey of shorefront residents a couple of years ago; the answers that respondants gave are the attached fields of the database.
    The yellow dots are the selected boats in this picture..
  2. Shoreline: This is a polygon coverage, representing shoreline, nada más nada menos...
  3. Bathymetry: This is bathymetric informatio of the water area, in a raster (grid) format.

Objectives: find the area of the inlet within discrete depth classes

In this exercise, we wish to find the total surface area of water in the inlet within discrete depth ranges (>1 ft., 1-2 ft., 2-3 ft., etc.).

the problem:

The data set as given is not suitable to readily determine the water depth classes. One problem is apparent when you click "off" the shoreline coverage. You will see that the bathymetry coverage is reporting depth values in many areas underneath the shoreline area, as shown in the following illustration:

the solution:

Somehow, we need to create a mask to "clip" out the area in the bay, like a cookie cutter. Ideally, we could use the shoreline coverage. However, one problem exists with the shoreline, is that it is telling us where the shore is. What we need is a coverage telling us where the shoreline is not. We need, somehow, to create a coverage that is the negative image of the shoreline; we need a polygon that covers the water area.

Creating a polygon for the water area

There are several good ways to create a polygon using the shore boundaries. This is just one way to do it.

step 1: copy the shoreline coverage to your directory

You will need to do this step to be able to edit anything.

step 2: add some arcs to close off open areas

Here, we can add a few arcs to the mouth of the bay and the river reaches to "close off" the water there. This can be done in arcedit, however, it will be quicker and ultimately easer to do the edits onscreen in arview.

To do this in arcview, it is necessary to first convert the coverage to a shapefile that resides in a directory where you have write permission. Assuming that your working directory is /home/gis**/lab2:

Your screen should appear thus:

Now you can open the coverage for editing:

start with a single mouse click, then follow the entire boundary with single mouse clicks until a polygon is drawn. Your view will appear as such:

Each vertex is a single mouse click. When you reach the last vertex, finish the entry with a double mouse click. Your image will look like this-- don't panic!

The polygon you just added has covered the entire area of interest. Simply stop editing:

When you stop editing, your new polygon will be yellow. Why?

When you have finished with the shapefile, take a look at your working directory. By going to a command shell, and typing ls, you will someithing like the following:

Remember that a shapefile is a collection of files. These files you see are all the files requires to define the shapefile shoreline. This file has the original shoreline, plus the arcs forming the polygon that we added in the last step. However, the arcs are simply "on top" of the shoreline arcs that existed before (a condition known as "spaghetti digitizing"). We need to find a way to split all of these arcs at the intersections; once that is done, we will have a polygon made up of the boundary of the water area.

The Arc/INFO clean command splits arcs at intersections quite nicely. If we convert this shapefile to an Arc/INFO coverage, then we can use clean to split all arcs at intersections.

Now vew the results through arcview. View->Add Theme: shoreline

Now you have a coverage in your window that has several new polygons. If you select the one in the center, you can save it to a separate file as a new coverage.

Paul Box
Last modified: Thu Feb 4 17:49:02 MST 1999