How To Use the Memphis Hazard Map Data

Creating a Surface from Points

The surfaces created for the maps in the Graphical Format section were produced by interpolating the point shapefiles that are available for download on this site.

There are several methods of interpolation available in Spatial Analyst and a brief description is provided below. More information is available in ArcGIS Help.For this series of maps, spline interpolation was used.

A brief overview of the ArcGIS Interpolation toolset

Surface interpolation functions create a continuous (or prediction) surface from sampled point values. The continuous surface representation of a raster dataset represents height, concentration, or magnitude (for example, elevation, pollution, or noise). Surface interpolation functions make predictions from sample measurements for all locations in a raster dataset whether a measurement has been taken at the location or not. There are a variety of ways to derive a prediction for each location; each method is referred to as a model. With each model, there are different assumptions made of the data, and certain models are more applicable for specific data (for example, one model may account for local variation better than another). Each model produces predictions using different calculations.

The Inverse Distance Weighted (IDW) and Spline methods are referred to as deterministic interpolation methods because they assign values to locations based on the surrounding measured values and on specified mathematical formulas that determine the smoothness of the resulting surface. A second family of interpolation methods consists of geostatistical methods (such as Kriging), which are based on statistical models that include autocorrelation (the statistical relationship among the measured points). Because of this, not only do geostatistical techniques have the capability of producing a prediction surface, but they can also provide some measure of the certainty or accuracy of the predictions.

Tool Description
IDW Interpolates a surface from points using an inverse distance weighted technique.
Kriging Interpolates a grid from a set of points using kriging.
Natural Neighbor Interpolates a surface from points using a natural neighbor technique.
Spline Interpolates a surface from points using a minimum curvature spline technique.
The interpolated grids were then displayed by drawing raster stretching values along a custom color ramp.

Ramping colors in the Layer Properties dialog box:

  1. Right-click the layer in the table of contents that shows a quantitative value and click Properties.
  2. Click the Symbology tab.
  3. Click Quantities and click Graduated colors.
  4. Choose a Value field and a Normalization field if desired.
  5. Set the number of Classes or change the classification scheme by clicking Classify.
  6. Double-click the top symbol in the list and set the start color for the ramp.
  7. Click OK.
  8. Double-click the bottom symbol and set the end color.
  9. Click OK.
  10. Optionally, double-click any middle symbol to set its color.
  11. This lets you create a multipart color ramp.
  12. Click all the middle symbols for which you've set the color.
  13. By selecting one or more middle symbols, the color of those symbols is included in the new ramp; otherwise, ArcMap only uses the top and bottom symbols.
  14. Right-click a symbol and click Ramp Colors.
  15. Optionally, if you want to use the new color ramp on another layer, right-click the Color Ramp dropdown and click Save to style to save your new ramp to your default style.
  16. Click OK.

To recreate the color ramp used in this map series, use the following fourteen algorithmic color ramps:

Dark Navy, Ultra Blue
Ultra Blue, Lapis Lazuli
Lapis Lazuli, Logo Blue
Logo Blue, Deep Forest
Deep Forest, Moss Green
Moss Green, Olivenite Green
Olivenite Green, Citroen Yellow
Citroen Yellow, Autonite Yellow
Autonite Yellow, Seville Orange
Seville Orange, Flame Red
Flame Red, Mars Red
Mars Red, Poinsetta Red
Pointsetta Red, Medium Coral Light
Medium Coral Light, Rose Quartz

Or, download Memphis.style and place in C:\Program Files\ArcGIS\Bin\Styles. When you go to the Style Manager you will see Memphis on the list. Select it and it should then be available in the selection of color ramps in Layer Properties => Symbology => Stretched.

Then edit the high/low values for all data sets to 0.99 high and 0.01 low. Otherwise the colors will be stretched only through the range of the current data set rather than the range of all the datasets.