DNR Sample Generator Extension

DNR Sampling Generator Extension

Purpose of this Extension

The ArcView Sampling Extension contains tools to help researchers and managers develop random and systematic point and transect samples for selected graphics or polygon features in a shapefile. For point samples the X and Y coordinates of the points are written as attributes to the shapefile and for Transect samples the X and Y coordinates of the transect endpoints are recorded. These coordinates can then be uploaded to a GPS for guidance in the field.

 

Functionality Highlights

At this time this extension contains the following functionality:

Random Point Sample	Generates a shapefile of randomly placed points within selected polygon features or graphics. If more than one polygons are selected the user can treat them as a single, multi-part polygon or as individual polygons. For example, if the user wants to create 50 samples and they have 5 polygons selected, do they want to create 50 samples per polygon for a total of 250 samples or do they want to spread 50 samples across 5 polygons?
Systematic Point Sample	Generates a shapefile of systematically placed points for the selected polygon features or graphics based on user supplied X and Y spacings or based on a desired number of points. Users can also specify an angle for the Grid and also define a random starting point or a user input coordinate.
Triangular Point Array Sample	This option generates a shapefile of points within selected polygon that are spaced based on a user supplied distance, much like the Systematic Point Sample tool. The difference is that every other row of sample points is offset 1/2 the sample distance to form a triangular shaped pattern across the View. Credit goes to Nick Seigal for contributing this code - Thanks!.
Sample Points Along Outline	This option creates a set of sample points along a polygon outline. There are two options for placing points, you can specify the total number of points that are evenly spaced across the outline or you can specify a distance between the points.
Hexagonal Polygon Sample	This option generates a set of adjacent hexagonal polygons within the selected polygon shape/graphic. You supply the distance between sample centers. The output is similar to the EPA hexagonal sampling methodology. You can choose to have the hexagons overlap or be completely contained within the selected Polygon shapes/graphics. Credit goes to Nick Seigal for contributing this code - Thanks!.
Transects	Generates a shapefile of randomly placed transects (both position and angle) within the selected polygon features/graphics. User supplied inputs include defining whether or not the transects can overlap, the sampling width, minimum and maximum transect lengths and specify a number of transects or a percent of area.
Random Segment Sample	This option creates a shapefile of randomly selected linear segments from an input line theme. User input includes the number of segment samples that you want and the length of the sample. An example would be to create a set of line features from an input theme of streams that are one mile in length. Output features may be a set of contiguous lines that are merged together in order to get the proper segment length.
Systematic Transect Sample	This option creates a set of transects that are equally spaced across a set of selected polygon shapes/graphics. User input includes the distance between transects and the angle of the transect. You could create a set of transects that are 10 kilometers apart at a compass angle of 45 degrees for example, and then load these transect shapes into your GPS. Users can also split each of the transects into lengths specified by the user.
Create Acres Grid Cell Graphic	Creates a grid graphic on the view for the selected polygon graphic or shape. The user specifies the area of the cells in Acres and a graphic is generated and placed on the view document.
Select Features Randomly	This option works on point, line or polygon themes and allows the user to randomly select a percentage of features within a shapefile. These features could then be written to their own shapefile for further analysis.

For random sampling points are placed in the selected polygons or graphics. These points will fall within the polygons and the user has the option of specifying the number of points, the sample radius and the minimum distance two points can be together (default = SampleRadius * 2). Point Attributes include a sample ID number and the X and Y coordinate of each sample point.

 

For Systematic Point Samples the user parameters include the X and Y spacing between points and the minimum distance from the polygon boundary. Attributes include a sample ID number, the row and column of the entry and the X and Y coordinates.

 

For Transects the user has the ability to define the various parameters of the transects generated including:

• Minimum Transect Length
• Maximum Transect Length
• Transect Sample Width
• Minimum Transect Spacing
• Do Transects get to Overlap?
• Number of transects OR the percent of area covered by the transect sample.

Transects are created using the following rules:

1. The starting point of the trasect is determined by randomly placing a point within the polygons of interest.
2. Then an angle for the transect is randomly determined.
3. A transect line is created from the staring point that is the length of the Maximum transect length parameter. 
4. The transect is the clipped to the input polygons so that the sample area for that transect does not extent outside the polygon of interest. 
5. If the user has chosen to not allow overlapping transects AND the line overlaps another transect the line is thrown away and the process goes back to step (2). 
6. The process is repeated until a) the number of transects desired is generated or b) until the area of the sample meets the percentage criteria entered by the user. 

Attributes for Transects include a transect ID, Azimuth,Length,Sample Area, Transect Start X,Y and Transect End X,Y.

 

For Grid Cell Graphics the user has the option of specifying the size of the grid cells in acres. The grid cells are then created for the selected set of polygons or polygon graphics.

 

Known Issues

• When creating transects make sure you don't ask for too much. This process can take a really long time if you choose to make too many transects or to have too much area occupied by the sample. This can be compounded if you don't allow transects to overlap. Be patient, it should finish.
• Same thing with the Random Segment Sample, there are cases where it's problematic to create the samples based on the user inputs.
• All of these routines work on projected shapefiles. Un-Projected shapefiles in a projected view is not supported.

Credits

• Thanks to Chuck Loesch, US Fish and Wildlife Service in Bismarck North Dakota, for helping with the concepts, decision making process, and documentation for this extension.
• Thanks to Nick Seigal for providing the base code for the Triangular Point Array sample and the Hexagonal Polygon Sample Routines. Much appreciated Nick!

History

Version 2.1 - 7/6/2004 Modifications made at this release...

• Added an option Triangular Point Array Sample that accesses a script called Sample.MakeTriangularArray. This script creates an array of points that are separated by a user supplied sample distance. Each successive row of points is offset by 1/2 the sample distance. This script uses an existing polygon theme as a base.
• Added a option called Hexagonal Polygon Sample that access a script called Sample.MakeHexagonalArray. This script creates an array of hexagons that touch based on a user supplied sampling distance. This script uses an existing polygon theme as a base.
• Added an option called Systematic Transect Sample which creates a set of regularly spaced transects across a selected polygon or graphic. The user inputs the distance between transects and a compass angle 0 - 90 degrees. Creates a shapefile.
• Add an option called Select Features Randomly that selects a percentage of the features in a theme.

Version 2 - 4/7/2004 Modifications made at this release

• the addition of a script that randomly selects segments of a linear network of a particular length. Script name is Sample.LinearSegments.

Version 1.1 - 5/7/2003

• Modified the Random Point generator so that the user has the ability to specify whether they want multiple polygons treated as a multi-part shape or as a set of individual polygon areas. For example, if the user wanted to create the same number of sample points for each polygon in a set of selected polygons or graphics. Version 1.0 would group the selected polygons and distribute the desired number of sample points across them rather than for each area. Now the user has both options. Thanks to Jeremy Bell from the Massachusetts Division of Fish and Wildlife for the suggestion.
• Added a new command, Create Acres Grid Cell Graphic that creates a graphic of grid cells with a user specified area in acres.

Download Program

DNRSAMPLE.ZIP - Sampling Extension with Documentation (2 mb)

DNRSAMPLE.PDF - Sampling Extension Documentation (Adobe .PDF - 1.2 mb)

If you would like to be notified of future MNDNR GIS extension upgrades please enter your email address below and press "Submit".

E-mail Address: 

Back to ArcView Extensions Page | Back to ArcView Resource Page