The Waikato Regional Council uses aerial photography and a point sample technique to measure soil stability. This consists of one hectare sample areas spread over a 2km grids across the Waikato region.
The monitoring occurs every five years as aerial photography becomes available.
This point sample survey was completed for the first time in 2002 and repeated with aerial photography from 2007.
Point sample analysis is a statistical technique that has long been applied in the natural sciences, to extract sample data from field sites, aerial photographs, or maps. Aerial photographs are visually inspected by someone who has landform and erosion knowledge and skills in photographic interpretation.
Within each sample area, the land stability and any soil disturbance as a result of land use or natural processes are recorded. Soil disturbance is measured by interpreting the one hectare sample area and recording any sign of disturbance. Wherever soil disturbance was recorded within a sample area, a cluster analysis is used to record the percentage of bare ground within each of those sample areas. This determines the percentage of soil bare in the one hectare sample area.
Although the 2 km grid is not spatially random, it does provide a random sample because the underlying landforms, soils and land uses are distributed irregularly across the region. In addition, the 2 by 2 km spacing provides sufficient sample areas to represent the whole of the region.
Burton AS, Taylor A, Hicks DL 2009. Assessing soil stability. In: Land and soil monitoring : a guide for SoE and regional council reporting. Hamilton, Land Management Forum. pp. 89-116
The raw data is stored in GIS layers
A standard set of codes for photo interpretation has been developed to cover the main land use classes, surface type classes, land use disturbance classes and natural process classes.
A unique reference number for each sample point, from 1 to 6122 was automatically assigned by the GIS program Geomedia. However because this number may change if the attribute table is re-opened in a different workspace, a locked identification number was assigned by an EW GIS specialist to each point, enabling their re-location during data sorts and analyses.
Sample point locations were stored as Geomedia metadata. These are cross referenced to a screen display (shapefile) which shows their locations relative to region-wide orthophotos and map layers e.g. topographic; and a database (attribute table) which contains data recorded for all points. It is duplicated in an Excel spreadsheet which enables data sorting.
Sorts and point counts
An initial data sort was carried out in the Excel spreadsheet, to check for consistency in use of codes, and correct where necessary. Subsidiary spreadsheets were created region-wide and for each land use e.g. dairy pasture. These were repeatedly sorted to count points in each category of interest i.e. soil stability, nature of disturbance.
Formulae were inserted into each spreadsheet, enabling calculation of percentages and error margins for each category of interest.
Sort, point count and statistical analysis results for particular topics were stored as four series of summary spreadsheets.
Two reports were supplied.
Soil Stability in the Waikato Region 2007
Changes in Soil Stability and Disturbance from 2002 to 2007
Cluster sampling is used to measure the percentage of bare soil. It entails recording the incidence of bare soil at each of 100 dots set in a 10 x 10 grid within a one hectare area around the sample point. The number of dots with bare soil is recorded as an attribute.
Photo interpretation error is ascertained by randomly selecting 100 points and assessing them in the field against the photo interpretation results. They indicate reliability of photo-interpreted data stored for the points in the sample. For the completed surveys to date photo-interpretation accuracy has typically been in the 85% to 95% range. Most errors are simply confusion of land use etc. with another that is similar.
None relevant to this indicator.
Geomedia is used to display and store the data from photo interpretation. The data is then exported to Excel where the analysis procedure is to be sorts either by database query or transfer to spreadsheets, followed by point counts and conversion of totals to percentages of regional area, area in each land use, or area with each soil conservation cover (as appropriate). Significance tests are to be applied, to calculate confidence limits for results. The indicator is compiled using the data from the point samples.
This indicator is not an actual measure of soil erosion. It is a point sample method that provides regional data on soil stability and uses cluster analysis to provide a measure of bare ground. The point sample analysis technique has been designed to provide statistical data for regions. Region-wide samples are sufficiently large that they can also provide valid data for reasonably large subdivisions within a region. However, data analysis will not be reliable for soil stability/disturbance in an area of land any smaller than 100 km2.
No changes are planned for this indicator.