FOREST INVENTORY & ANALYSIS
SAMPLE FRAME
Ken Winterberger
PNW Research Station
Interior Alaska Inventory Workshop
5–6 February 2014
Resource
Sampling
in Alaska
• Systematic vs. Stratified
• The hexagonal grid described – very briefly
• Application of The Grid in Alaska
Advantage —
Stratified
Sampling
• Can result in higher precision within a strata
• Can be more efficient
Disadvantage —
•
•
Not useful when population cannot be easily
partitioned. What are the strata?
Very difficult to use in monitoring due to
potential (expected?) changes in strata, e.g.,
vegetated becomes unvegetated or vice versa.
Stratified Sample —
Problem with changing
strata and allocation of
ground sample
Advantage —
• Representative of population
• Can be easy to locate samples
• Can be easy to intensify/de-intensify sample
Systematic
Sampling
Disadvantage —
•
•
•
Assumes that population is random —
it is not truly a random sample
Possibility of systematic sample
interacting with a hidden periodic
trait within the population
No methods available to reliably
estimate variance — assumption
that sample is random
The Grid
FIA sample frame based on the US
EPA’s Environmental Monitoring
and Assessment Program (EMAP)
Sampling Grid
• Sampling from grid allows
for sampling of any spatially
distributed and well defined
resource.
• Provides a consistent,
regular, spatial and
temporal distribution of
sampled locations.
• A regular systematic grid
best achieves a
randomization and equal
area sampling criteria.
• Using a truncated
icosahedron (soccer ball) as
the basis for the
development of the sample
grid allows for grid
intensification using factors
of 3, 4, 7, and 9.
• The originally described
EMAP sampling frame
(early ’90s) was developed
with the conterminous U.S.
and Alaska (mostly) fitting
into unique hexagons.
• If this ‘U.S. centric’
truncated icosohedron is
applied to the globe
unmodified, countries/
continents may not be
conveniently located within
a hexagon (e.g., Australia)
• In 2001 an application was
developed to allow the
creation of conveniently
placed hexagons, e.g., the
Australian hexagon
EMAP GRID PROGRAM
Available hexagons — and you can create your own if you
have the fortitude …
Enhancing (intensification) and
reducing (de-intensification) of
the EMAP Grid.
• Varying spatial distribution
of ecological resources
require that the sampling
grid intensity be adjustable
while preserving the regular
triangular grid structure.
• Adjustability is provided by
a hierarchical structure
imposed upon the grid.
Enhancing (intensification) and
reducing (de-intensification) of
the EMAP Grid.
•
•
•
•
Intensifying by factor of 3
Intensifying by factor of 4
Intensifying by factor of 7
Intensifying by factor of 9
Application of ¼ Intensity Grid
• BLM — 38% forest,
62% nonforest
• DOD — 89% forest,
11% nonforest
• FWS — 41% forest,
59% nonforest
• NPS — 21% forest,
79% nonforest
• TVSF — 100%
forest, 0% nonforest
• FS — 51% forest,
49% nonforest
• Major Land
Managers — 37%
forest, 69%
nonforest
• The Shebang —
38% forest, 62%
nonforest
• Systematic vs. Stratified
• The hexagonal grid described
– very briefly
• Application of The Grid in
Alaska