logo Calflora Grid Survey Design
Updated July 23, 2020
Introduction Data Model Software Example Notes

Introduction
As described here, Grid Survey (GS) is both weed mapping technique, and a proposed extension to Calflora's Weed Manager system. It is largely inspired by a presentation given by Morgan Ball and Katrina Olthoff at the 2019 Cal-IPC Symposium.

Currently Weed Manager supports various population based techniques of weed mapping. Typically, an agency keeps track of a particular patch of weeds by

  1. drawing a polygon around it;
  2. recording attributes (gross area, percent cover, number of plants) to assess its size; and
  3. treating it periodically.
The system stores all assessments and treatments of a single patch in a history stack. By examining the stack, a user can see the increase or decrease of the population over time.

The GS technique is similar, but there is no longer a need for a polygon. Instead, the target area is divided into grid cells, and the containing grid cell becomes the polygon for any recorded assessment or treatment. In GS, the focus is on abundance within a cell, or absence from a cell, rather than on population. A population may occupy several adjacent cells.

The possible benefits of GS include:

  • Absence is cleanly quantifiable. If a weed is being assessed and treated inside of a management unit over several years, evidence of progress in the containment of that weed will be 1. more cells in which the weed is no longer found, and/or 2. less abundance of the weed in those cells where it is found.

  • The metric of progress is very similar for all weeds. It may be possible to compare the effectiveness of treatment plans for various weeds, even if some of the weeds are small and some large.

  • Training for field staff may be simpler, and produce more uniform results, than with a population based approach.
GS also poses some challenges:
  • Data generated is voluminous, with a great number of records documenting absence. The smaller the cell size, the more voluminous.
Data Model
Grid Survey Framework
framework id
name
plant list
cell size (side of a cell in meters)
set of cells defining the target area
Grid Survey
framework id
survey id
field work date span
crew-cell assignments
Cell Assessment
framework id
survey id
cell id
cell assessment id
date
for every plant on the framework plant list:
percent cover (whole cell)
number of plants (whole cell)
Cell Treatment
framework id
survey id
cell id
cell treatment id
date
plant
treatment details
Discussion

A Grid Survey Framework is atemporal; it describes the area and the plants that will be surveyed periodically. The cell size can be set to any number of meters. In typical situation, the user making a Grid Survey Framework will start with the polygon boundary of a management unit, and ask for a grid of cells to be generated to cover that polygon (see image below).

A Grid Survey is a survey event. It is associated with a Grid Survey Framework and describes a period of time during which field crews will look for the plants in the area. It also contains the assignment of crews to particular cells.

A Cell Assessment record is made by a field crew. They search the entire cell for plants on the plant list, and record percent cover and number of plants for each. When making a new record, the values of these two attributes start at 0. When the crew finds one of the plants in the cell, they set the values accordingly.

A Cell Treatment record is made by a field crew to document the treatment of a single plant inside of the cell. As in WM currently, treatment details are determined by the organization doing the survey.

Grid Survey and Current Weed Manager Record Types

If the plant list in the Grid Survey Framework contains 10 plants, then a Cell Assessment record is equivalent to 10 observations, one for each plant. The polygon for each of these records is the cell boundary.

A Cell Treatment record is the same as a current Weed Manager treatment record. The polygon for this type of record is the cell boundary.

Software
Below are pieces of software that need to be developed or adapted to support Grid Surveys.

Grid Survey Editor

A user can make a new Grid Survey Framework by choosing a plant list, an area (eg a management unit polygon), and a cell size. A grid of cells is generated to cover the selected area.

A user can make a new Grid Survey by selecting a Grid Survey Framework, setting the field work dates and assigning crews to assess and/or treat cells.

This application will display Cell Assessments and Cell Treatments when they are available. It can display and summarize the extent of each plant on the plant list. When more than one Grid Survey has been completed, it can compare the results of several Grid Surveys. For each plant, it can show which cells are increasing (percent cover or number of plants) and which cells are decreasing.

Phone Application

The phone application needs to be able to generate Cell Assessment records and Cell Treatment records. To do this it needs to be aware of the plant list in the containing Grid Survey Framework, as well as the particular grid cells assigned to a user / crew.

The phone application needs to be able to show the user Cell Assessment data from a prior survey, if a prior survey has been completed. When a user arrives in one of their assigned cells, they should be able to see percent cover and number of plants for each plant on the list from the prior survey.

Example
The target area is Ragle Ranch, a Sonoma County Park. With the Grid Survey Editor, the user makes a new Grid Survey Framework record, choosing a cell size of 40 meters (the side of a cell is 40m), and generating a grid that covers the polygon boundary of the park:

Also with the Grid Survey Editor, the user makes a new Grid Survey record for June 1 - 7, 2021. The user assigns the southernmost cells of the grid (outlined in red below) to crew #1:

A member of crew #1 sees the assigned cells in the phone application, so they know when they are inside one of their assigned cells:

Notes
Changing Cell Size

If the cell size is changed to something bigger, records of presence transfer cleanly, but records of absence do not.

If the cell size is changed to something smaller, records of absence transfer cleanly, but records of presence do not.

From Population to Grid Survey

Observations made with a population based technique can be viewed against a grid, and can also be imported into a Grid Survey to supply records of presence. The Grid Survey technique, however, is much better at producing records of absence.



Cell Identifiers

The example screen shots above show grids generated via UTM coordinates. A 20 character cell identifier contains easting, northing, zone and the length of the side of the cell in meters. For instance, 587720,4206280,10:40.