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| Subject: | Re: [gislist] Technical question: Areal accuracy standards? |
| Date: |
08/10/2007 03:44:55 PM |
| From: |
Gordon, Heather C. |
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Thank you to everyone who has responded so far. It's true, as Mr.
Bannerman points out below, that many geophysical datasets have
ephemeral or 'fuzzy' boundaries, but vector data deals with crisp lines.
However, when you're dealing with human-defined conditions, how do you
deal with accuracy? Here's an example at the point I'm trying to drive
towards:
Say you are working for a land management agency, and you are using two
datasets: land ownership (collected at 1:24,000), and wilderness areas
(unknown source materials and collection date: 'institutional memory'
gone when the last person to do GIS for them retired). They ask you to
quantify the land ownership by wilderness area. You do so, perhaps for
over 100 wilderness areas, through a simple clip. Because you don't know
the accuracy of the wilderness dataset, you don't report any significant
digits like they had been in some of their previous documents, but for a
given area, you find:
112 acres public ownership, 2 acres of private land.
That agency responds by saying they are absolutely sure that there are
no acres of private land, and they know for a 'fact' that the wilderness
area is 115 acres.
So, not only do you have a possible positional error in relation to the
ownership, but a possible total area error.
How do you handle this?
Thanks again, Heather
________________________________
From: Bruce.Bannerman@dpi.vic.gov.au
[mailto:Bruce.Bannerman@dpi.vic.gov.au]
Sent: Thursday, August 09, 2007 11:46 PM
To: Gordon, Heather C.
Cc: gislist@lists.geocomm.com
Subject: Re: [gislist] Technical question: Areal accuracy standards?
Heather,
This really needs to be considered on a case by case basis.
Some areal features may be describing geographic phenomena that can be
accurately represented in polygonal form e.g. Cadastral boundaries.
However, often areal features are used to describe phenomena that may
not have crisp, well defined boundaries, e.g. geology, soils, drainage,
vegetation, areas subject to inundation etc. Vector boundaries
describing these features are usually the result of an interpretation
and educated guess. These types of features may be better modelled as
raster surfaces.
Often the original phenomena is captured in vector form at a point in
time, e.g. the edge of a lake. However the lake shoreline will usually
vary in location depending on season, rainfall, snowmelt, drought etc.
Considering the above, some data sets could be accurately defined, while
many probably could not.
In the end it will still come down to validating your data against 'well
defined' points.
However, also consider how the location used to define the 'well
defined' points was originally defined.
Considering the above, it may not be meaningful to place an arbitrary
accuracy value on the dataset. An approach that many people use is to
make an educated assessment of the accuracy of the data by relating it
back to well defined points, but also to include Metadata that
adequately describes the data, including: what it is intended to
portray: how it was defined and captured: as well as its intended use.
The end user will then be able to assess the suitability of the data for
their particular use.
Bruce
---------------------------------------
Bruce Bannerman
IT Solutions Architect - GIS
Information Development Branch
Minerals and Petroleum Division
Department of Primary Industries - Victoria
Australia
"Gordon, Heather C." <HEATHER.C.GORDON@saic.com>
Sent by: gislist-bounces@lists.geocomm.com
10/08/2007 11:10 AM
To
<gislist@lists.geocomm.com>
cc
Subject
[gislist] Technical question: Areal accuracy standards?
Hi all,
Here's my my issue, for which I could use some perspective: we all are
pretty familiar with the FGDC standards for data accuracy, wherein a
point location has to be within so many feet of the true location at the
given scale. However, how does that translate into accuracy over an
area? Would that be dependent on the number of vertices (assume that
each point must be within the given distance of the true location)? Or
would it pertain to the 95% confidence interval
[http://www.fgdc.gov/standards/projects/FGDC-standards-projects/accuracy
/part3/chapter3: page 7].
If you have any books or online articles/webpages that would offer
clarification, I would be indebted.
Thanks for your help,
HC Gordon
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