Hi All,
My name is Kris Netchy and I'm currently a graduate student working on marine invertebrate biogeography. I'm doing my research at the Florida Fish and Wildlife Research Institute in St.Petersburg, Florida. Here, we have a specimen collection of about 90,000 lots of marine invertebrates. I'm planning to utilize these records to look at spatial patterns, but the first step is to go through the database and georeference all of the location data. A lot of these records are very old and the geographic coordinates are not very accurate. I'm a beginner at this and most of what I'm doing is self-taught, so I often have questions. My main question is about estimating error of these coordinates.
Can someone help me with this, by any chance?
Here is one example question I have:
One record has the locality description: 4 miles NW of Carysfort Reef Lighthouse. I have the coordinates for the lighthouse and I can use a ruler to determine a point about 4miles North of that point, but it's not going to be very accurate. I read that it would have about 22.5 degrees of directional uncertainty...but what does this mean in terms of distance? I'd like to quantify the error in terms of distance, so I can make a buffered point or polygon in ArcGIS.
Any insight you have would be wonderful! Thank you!
Kris
actinopyga@gmail.com
Newbie georeferencing
Nevermind. I figured that
Nevermind. I figured that particular example out on my own. Thanks anyway folks.
Enjoy.
Kris
Georeferencing Resources
Hi Kris,
Here is the "essential" list of resources for learning about retrospective georeferencing theory and practice:
Wieczorek, J. 2001. MaNIS/HerpNet/ORNIS Georeferencing Guidelines: Available online at http://manisnet.org/GeorefGuide.html
Wieczorek, J. 2001. Georeferencing Calculator: http://manisnet.org/gci2.html
Wieczorek, J. and D.A. Bloom. 2002 (revised 2001). Georeferencing Calculator Manual: http://goo.gl/G5RM9
Wieczorek et al. 2004. The point-radius method of georeferencing locality descriptions and determining associated uncertainty. IJGIS 18-8. pp745-767.
Spencer, C. et al. 2005. Georeferencing For Dummies: Available online at http://goo.gl/To7tO
Chapman, A.D. and J. Wieczorek (eds). 2006. Guide to Best Practices for Georeferencing. Copenhagen: Global Biodiversity Information Facility. Available online at http://www.gbif.org/orc/?doc_id=1288.
Darwin Core Georeferencing Terms: http://rs.tdwg.org/dwc/terms/index.htm#locationindex
Cheers,
John
Wonderful! I appreciate it,
Wonderful! I appreciate it, John!
Kris
Distance uncertainty
Hey there,
I have another question and I'm not seeing an answer to this particular example in any of the above sources.
I have read that to estimate distance uncertainty, you base it on the fractional part of the distance and divide the denominator by 1, so 0.75mile S of Bakersfield post office would have a distance uncertainty of 1/4 mile. And a whole number distance, such as 9 miles S of Bakersfield post office, would be 1mile.
So what if your distance is 1mile S of Bakersfield post office? 1/1 = 1mile. That can't be right though.
Any input would be great...thank you!
Kris
Distance uncertainty
Hi Kris,
Though the specific example is not given in the MaNIS/HerpNet/ORNIS Guidelines (http://manisnet.org/GeorefGuide.html#combinarions_of_uncertainties_directions), nor in "The Point Radius Georeferencing Locality Descriptions and Calculating Associated Uncertainty" (Wieczorek et al. 2006), an offset of 1 distance unit falls into the following category:
"For distances that appear as integer powers of ten (10, 20, 300, 4000), use 0.5 times ten to that power for the uncertainty."
Thus, for your example, the component of the uncertainty coming from the distance imprecision is 0.5 miles. Note that this is not at all the same as the final uncertainty, nor does it contribute in a linear fashion to it. The reason is that the locality type is "distance at a heading", in which the imprecision of the direction interacts with the imprecision of the distance as described in the Combining Uncertainties section of the MaNIS/HerpNet/ORNIS at http://manisnet.org/GeorefGuide.html#combinarions_of_uncertainties_directions.
I hope that helps,
John
Ah yes. Thank you John! I
Ah yes. Thank you John! I realized the answer to my question after I posted...which is what often happens...
Deb requested that I post the solution I came to regarding my first question.
Well, after reading all of the papers that John posted and looking at the math in 3.4.2 in Wieczorek et al 2004, I found that the best method is to assume that the lengths (4 miles in this case) are small enough to approximate the Earth as flat...and to use the Law of Cosines, which is:
c^2 = a^2 + b^2 - 2abcos(x)
a and b are both the distance of 4miles, x is the angle of 22.5 degrees, and c is what I solved for.
This gave me the most precise uncertainty associated with directional imprecision
Thanks again!
Kris
Kris,
Kris,
You might be interested in using some of the automated tools for georeferencing which also assist in determining uncertainty. Here is a link to your locality in GEOLocate:
http://www.museum.tulane.edu/geolocate/web/WebGeoref.aspx?v=1&Locality=4 miles NW of Carysfort Reef Lighthouse&State=FL&County=Monroe&georef=run
Here is a shortened version of that link, in case it is split up.
http://bit.ly/Ns9roG
In your example GEOLocate identified 4 miles NW of Carysfort Reef, but not the lighthouse. You can tell, by clicking on the point and looking at the pattern indicated in the popup box for that point Assuming you know where the light house is (I could not find it on any of the base layers), you could make adjustments as needed.
Hope that helps,
Nelson