Deriving and Using Coordinates with Herp Data

Dedicated exclusively to field herping.

Moderator: Scott Waters

Post Reply
User avatar
chris_mcmartin
Posts: 2433
Joined: June 8th, 2010, 11:13 pm
Location: Greater Houston TX Area
Contact:

Deriving and Using Coordinates with Herp Data

Post by chris_mcmartin » April 23rd, 2013, 4:21 pm

I whipped up this FAQ today and hope someone will find it useful!

Deriving and Using Coordinates with Herp Data (PDF; 851 KB)

Enjoy!

User avatar
Chris Smith
Posts: 2340
Joined: June 7th, 2010, 8:13 pm
Location: Minnesota

Re: Deriving and Using Coordinates with Herp Data

Post by Chris Smith » April 23rd, 2013, 4:27 pm

I must confess that I only skimmed it, but I highly recommend that people record GPS accuracy while taking waypoints. This is especially important if you plan or hope to share data with state or federal agencies (if you want to know why, PM me).

I believe the HERP - Mobile app records this automatically (if not, future updates likely will).

Great document! I know you put a lot of time into it... Thanks!

-Chris S.

User avatar
chris_mcmartin
Posts: 2433
Joined: June 8th, 2010, 11:13 pm
Location: Greater Houston TX Area
Contact:

Re: Deriving and Using Coordinates with Herp Data

Post by chris_mcmartin » April 23rd, 2013, 4:34 pm

Yeah, I hope people only list as many decimal places as accuracy allows...I do explain this somewhat in the PDF but did not get into Dilution of Precision, EPEH, etc., figuring that may be "beyond the scope" of most herpers' capability (or interest)! If you have information on agencies' requests for such information PM me and I may be able to incorporate it into the PDF.

That being said, I have a hunch many people, including data hounds and decision-makers at various agencies, might not understand the capabilities and limitations of GPS... 8-)

User avatar
Kevin Messenger
Posts: 525
Joined: June 7th, 2010, 2:00 pm
Location: Huntsville, AL
Contact:

Re: Deriving and Using Coordinates with Herp Data

Post by Kevin Messenger » April 23rd, 2013, 8:37 pm

I try to never hit my "mark" button unless the accuracy is at 9ft or less. I don't think many people pay attention. Accuracy is extremely important to me. If I have the time, I will always average the waypoints until the accuracy gets around 6 ft

User avatar
chris_mcmartin
Posts: 2433
Joined: June 8th, 2010, 11:13 pm
Location: Greater Houston TX Area
Contact:

Re: Deriving and Using Coordinates with Herp Data

Post by chris_mcmartin » April 24th, 2013, 2:33 am

Kevin Messenger wrote:I try to never hit my "mark" button unless the accuracy is at 9ft or less. I don't think many people pay attention. Accuracy is extremely important to me. If I have the time, I will always average the waypoints until the accuracy gets around 6 ft
Unless the species is notoriously sedentary, could you reasonably expect the animal to move more than 9 feet during the course of its activities? 8-)

Point taken about accuracy being subjective, though. I suppose the GPS signal's "within 10 feet 95% of the time" accuracy is fine for most applications (including most herping), but if working on a project covering a small area of fragmented habitat (e.g. to make a case against development) sometimes the difference a couple of feet makes may be critical.

You can always refine the coordinates using Google Earth or other mapping software (or a paper chart!) using the initial GPS waypoint as a guide--for example, if your saved coordinates are supposed to be a particular boulder, but they're off a couple of feet when plotted in Google Earth, adjust accordingly (provided the boulder is visible in the imagery).

User avatar
Don Becker
Posts: 3355
Joined: June 7th, 2010, 3:21 am
Location: Iowa
Contact:

Re: Deriving and Using Coordinates with Herp Data

Post by Don Becker » April 24th, 2013, 5:01 am

Kevin Messenger wrote:I try to never hit my "mark" button unless the accuracy is at 9ft or less. I don't think many people pay attention. Accuracy is extremely important to me. If I have the time, I will always average the waypoints until the accuracy gets around 6 ft
Even at times when I was forced to record a point with less accuracy, when I plotted the point on a map, it was spot on to where I was standing. I just bought a P&S camera with GPS built in too. It doesn't tell you what the accuracy is, so I took it out to do some tests the other day. I put the camera on my lap while driving, and just hit the shutter button everytime I went through an intersection. When I plotted the points in google earth, they were all right in the intersection, even in the right lane of the road. GPS units are getting better every day. I would wager the chips in most cell phones now are better than the ones in dedicated gps units that are a few years old.

User avatar
Gluesenkamp
Posts: 290
Joined: June 8th, 2010, 6:57 am
Location: Texas

Re: Deriving and Using Coordinates with Herp Data

Post by Gluesenkamp » April 24th, 2013, 5:11 am

Excellent tutorial, Chris*. Another issue is that folks should always indicate the accuracy of the observation. This is easy to do using the methodology you presented. I run across this issue all the time with the Texas natural Diversity Database. Indicating a point on a map without some indication of resolution results in a huge loss of information. A location such as "Freer, TX" results in a buffer that encompasses the entire town (and may not be correct anyway). I'm not sure about NAHerp but in inaturalist, you can indicate the locational accuracy in the drop down box for each observation. That allows us to plot element occurrences with appropriate buffers. Photos taken with smart phones usually have metadata associated with them that makes this super easy but not everyone takes pics with their phone (thank goodness).
Andy

*Would you mind cross posting this to the H.O.T. project as a journal entry?

User avatar
chris_mcmartin
Posts: 2433
Joined: June 8th, 2010, 11:13 pm
Location: Greater Houston TX Area
Contact:

Re: Deriving and Using Coordinates with Herp Data

Post by chris_mcmartin » April 24th, 2013, 6:49 am

Gluesenkamp wrote:*Would you mind cross posting this to the H.O.T. project as a journal entry?
I wanted to, but was unsure if the Average User could post journal entries...didn't see a means by which to do so. All the existing posts are by you or other project admins.


Regarding the accuracy, the Herps of Texas project (and I presume all projects within iNaturalist) lets the user select the circle of accuracy, which is a fairly user-friendly method. Some GPS units do not display their Estimated Position Error Horizontal/Dilution of Precision, and on those that do, not everybody checks it. Cross-referencing a map when entering coordinates, as on Herps of Texas, should make it easy to ascertain "yes, I was within this radius."

User avatar
Tom Lott
Posts: 43
Joined: July 12th, 2010, 2:29 pm
Location: south-central Texas
Contact:

Re: Deriving and Using Coordinates with Herp Data

Post by Tom Lott » April 24th, 2013, 7:22 am

Excellent tutorial. Thanks for posting it.

User avatar
Kevin Messenger
Posts: 525
Joined: June 7th, 2010, 2:00 pm
Location: Huntsville, AL
Contact:

Re: Deriving and Using Coordinates with Herp Data

Post by Kevin Messenger » April 24th, 2013, 10:48 am

chris_mcmartin wrote:Unless the species is notoriously sedentary, could you reasonably expect the animal to move more than 9 feet during the course of its activities?
Yes, without a doubt. I don't think of a coord as it's movement, I like visualizing coords as "it was at this stump, it was at this piece of tin, it was beside this clump of wire grass." Along similar lines, when I am doing recreation herping, I've stopped taking coords of black racers. Because on the roads I cruise, they literally could be anywhere, whereas the other species tend to have "hotspots" for X-species. If I took coords of every racer, the entire stretch of road would be a racer hotspot. Something I already know.
chris_mcmartin wrote:You can always refine the coordinates using Google Earth or other mapping software (or a paper chart!) using the initial GPS waypoint as a guide--for example, if your saved coordinates are supposed to be a particular boulder, but they're off a couple of feet when plotted in Google Earth, adjust accordingly (provided the boulder is visible in the imagery).
Yeah, usually when I do stuff, and if I want to revisit a stump for example, a few feet will matter. I've tried to find friends' waypoints from time to time. I get to the "point" - looking for a specific log, or what have you, and it's nowhere to be seen. It could be over a hill, and if that's the case, then you gotta wonder which log. So when I want to retrace my own steps/ spots, the more accurate I can get that number the better. But for other things like road cruising, marking a DOR - 9ft accuracy is plenty accurate in my opinion. Most roads are wider than 9 ft. So yeah, it all depends on the situation. But, as you pointed out, I do frequently adjust misplaced waypoints via GE. I also trust GE's estimation of elevation much more than my GPS' estimation.
psyon wrote:Even at times when I was forced to record a point with less accuracy, when I plotted the point on a map, it was spot on to where I was standing. I just bought a P&S camera with GPS built in too. It doesn't tell you what the accuracy is, so I took it out to do some tests the other day. I put the camera on my lap while driving, and just hit the shutter button everytime I went through an intersection. When I plotted the points in google earth, they were all right in the intersection, even in the right lane of the road. GPS units are getting better every day. I would wager the chips in most cell phones now are better than the ones in dedicated gps units that are a few years old
That is very awesome. I had always wondered about those and their accuracy, glad someone went out and did such road tests. But I suspect the biggest difference would be when dealing with terrain and canopy cover. Out in the open, I can imagine those cameras performing well. But I would be curious to see how they do in a ravine, or some place where my garmin typically get's +/- 30 ft - at which point I do the average waypoint feature in order to bring it down to 9-12ft.

For a future GPS, all I'd like to see is a faster accurate response time. Turn it on, within seconds it's down to less than 20 ft (even in dense canopy or in a ravine). It would be really nice to get down to 1 m accuracy as the norm, but I think the military says they purposely limit accuracy in GPS units because they don't think civilians needs such accuracy (of course with bigger GPS units you can change that - and using the average waypoint feature, I got my Garmin 60Csx down to 2 ft accuracy).

User avatar
kaptainkory
Posts: 187
Joined: June 28th, 2010, 10:33 pm
Location: Bella Vista, Arkansas

Re: Deriving and Using Coordinates with Herp Data

Post by kaptainkory » April 24th, 2013, 11:05 am

I only skimmed as well. While I think you have noble intentions, I don't think they are very realistic. Any agency compiling records from multiple sources, collected by multiple means should not reasonably expect to get "boulder-specific" data (regardless of how many decimal places are recorded). If they are basing their fine-scale conclusions off such data with that expectation, they aren't very bright!

The only reliable way to get that level of precision is to put boots on the ground with people TRAINED in a SPECIFIC PROTOCOL.

Specifically to your document... If people just need to convert from one format to another (and don't really care about understanding HOW/WHY), there are online converters...much easier. I have a strong preference for working in decimal degrees, WGS 84 (same as Google Earth).

User avatar
Gluesenkamp
Posts: 290
Joined: June 8th, 2010, 6:57 am
Location: Texas

Re: Deriving and Using Coordinates with Herp Data

Post by Gluesenkamp » April 24th, 2013, 11:17 am

My agency certainly doesn't strive for "boulder-specific" data but +/- 50m is a vast difference from +/- 1km, especially when you are considering making someone move a pipeline. Cavers certainly strive for (and achieve) "boulder-specific" precision but caves are typically singular, non-moving entities. That's where good notes and ample use of comment fields come in handy.

User avatar
kaptainkory
Posts: 187
Joined: June 28th, 2010, 10:33 pm
Location: Bella Vista, Arkansas

Re: Deriving and Using Coordinates with Herp Data

Post by kaptainkory » April 24th, 2013, 11:48 am

Gluesenkamp wrote:My agency certainly doesn't strive for "boulder-specific" data but +/- 50m is a vast difference from +/- 1km, especially when you are considering making someone move a pipeline.
Then I presume you would have some reasonable way of KNOWING the level of precision (reliable collector, availability to confirm with the collector, double-checking that the habitat for the record is "right" for the species, etc.) before you would base tangible decisions upon it. If so, that's the same point I was making. If you DON'T KNOW, but are still taking real physical actions (moving a pipeline), seems like you are setting your agency up for criticism and controversy.

User avatar
kaptainkory
Posts: 187
Joined: June 28th, 2010, 10:33 pm
Location: Bella Vista, Arkansas

Re: Deriving and Using Coordinates with Herp Data

Post by kaptainkory » April 24th, 2013, 12:10 pm

Just to maybe clarify my position... I do agree that if you KNOW the level of precision (+/- 5m, for example), then by all means this should be included WITH any reported coordinates. I guess where I'm disagreeing is that the number of decimal places should be the way to indicate the level of precision in this case. Scientifically, yes, this is how it is done, but realistically...no. Many measurement devices (GPS units, phones, cameras, etc.) do not correctly display/record the number of decimal places as an indication of precision. If coordinates (of any decimal length) are provided WITHOUT the additional level of precision, I think an agency should not reasonably ASSUME the level of precision from the number of decimals. And except in specific cases, it's probably an overburden to expect a collector to sweat the significant figures of their GPS coordinates.

User avatar
chris_mcmartin
Posts: 2433
Joined: June 8th, 2010, 11:13 pm
Location: Greater Houston TX Area
Contact:

Re: Deriving and Using Coordinates with Herp Data

Post by chris_mcmartin » April 24th, 2013, 12:39 pm

Kevin Messenger wrote:I think the military says they purposely limit accuracy in GPS units because they don't think civilians needs such accuracy (of course with bigger GPS units you can change that - and using the average waypoint feature, I got my Garmin 60Csx down to 2 ft accuracy).
You are referring to Selective Availability (the ability of the military to get higher-precision coordinates than civilians), which was turned off by President Clinton in the 90s after it became readily apparent that airliners and basically the entire world was increasingly turning to GPS coordinates for a variety of reasons.

I can't speak to the logic/algorithms utilized in the Garmin unit...GPS signals as I mention in this thread and in the PDF are accurate to within 3 meters (~10 ft) 95% of the time. Sometimes it might be better but it generally won't be worse (except for that 5% of the time). A lot of this depends on which satellites are visible from your location. Even the Wide Area Augmentation System (WAAS), which many newer GPS units can utilize (no extra equipment required on the user's part) simply incorporates a ground-based signal to help correct for system timing errors, atmospheric attenuation, etc. and still just helps guarantee that 10-foot accuracy.

I suspect the 60Csx arrives at 2-ft accuracy by comparing the GPS satellites' own estimated signal errors and averaging the "better than 10-foot accuracy" readings it gets over a short period of time. Do you ever run into a situation (location and/or time of day) where it just can't achieve that accuracy?


kaptainkory wrote:I only skimmed as well. While I think you have noble intentions, I don't think they are very realistic. Any agency compiling records from multiple sources, collected by multiple means should not reasonably expect to get "boulder-specific" data (regardless of how many decimal places are recorded). If they are basing their fine-scale conclusions off such data with that expectation, they aren't very bright!
I'm not writing from an agency's perspective, but from a user hoping to provide meaningful data. Based on feedback in this thread, I may need to update the PDF to include recording the level of accuracy.

Basically, what I probably need to clarify is that you should ONLY record to decimal-place X depending on your chosen means of displaying the information (degrees-decimal, degrees-minutes-decimal, etc).

Because of my background/profession, I'm most comfortable with degrees-minutes-decimal, because each minute of latitude is one nautical mile (6076'). Longitudinal degrees converge at the poles, hence they do not represent a constant unit of distance, but can be approximated in the US to be just under one statute mile as I discuss. I have to do a little mental math to convert degrees-decimal in my head if I'm using that format in the field to estimate position and distance traveled.

Many measurement devices (GPS units, phones, cameras, etc.) do not correctly display/record the number of decimal places as an indication of accuracy.
If you did more than skim, Kory, you'd know I already addressed that... :P :beer:

User avatar
chris_mcmartin
Posts: 2433
Joined: June 8th, 2010, 11:13 pm
Location: Greater Houston TX Area
Contact:

Re: Deriving and Using Coordinates with Herp Data

Post by chris_mcmartin » April 24th, 2013, 12:41 pm

I hope I don't come across as overly defensive in my replies...I appreciate the feedback by all!

User avatar
Carl Brune
Posts: 486
Joined: June 7th, 2010, 10:22 am
Location: Athens, OH
Contact:

Re: Deriving and Using Coordinates with Herp Data

Post by Carl Brune » April 24th, 2013, 1:55 pm

I personally don't see much need for accuracy better than 30' and don't see much point in recording the (estimated) error so long it has reached this level. I can also appreciate that some applications / some people / some beancounting agencies may have different needs / opinions.

One thing that is important is to do independent checks, or at least spot checks. My old gps, an early garmin etrex from over 10 years ago, used to make make big mistakes (like 1/4 mile) once in a while (like 1 out of 200 points). I almost always mark the position of my car when I go for a hike, and can usually confirm that point easily with google earth or similar.

User avatar
Don Becker
Posts: 3355
Joined: June 7th, 2010, 3:21 am
Location: Iowa
Contact:

Re: Deriving and Using Coordinates with Herp Data

Post by Don Becker » April 24th, 2013, 6:03 pm

Kevin Messenger wrote:But I suspect the biggest difference would be when dealing with terrain and canopy cover. Out in the open, I can imagine those cameras performing well. But I would be curious to see how they do in a ravine, or some place where my garmin typically get's +/- 30 ft - at which point I do the average waypoint feature in order to bring it down to 9-12ft.
I will let you know as the weather warms up :) If I don't make a post about it, send me a PM in June :P

User avatar
Gluesenkamp
Posts: 290
Joined: June 8th, 2010, 6:57 am
Location: Texas

Re: Deriving and Using Coordinates with Herp Data

Post by Gluesenkamp » April 25th, 2013, 4:49 am

Chris,
Perhaps a discussion of *accuracy* versus *precision* is in order?

User avatar
chris_mcmartin
Posts: 2433
Joined: June 8th, 2010, 11:13 pm
Location: Greater Houston TX Area
Contact:

Re: Deriving and Using Coordinates with Herp Data

Post by chris_mcmartin » April 25th, 2013, 6:18 am

Gluesenkamp wrote:Perhaps a discussion of *accuracy* versus *precision* is in order?
If I were to try to explain it, I think it would confuse even more people than I already have!

http://en.wikipedia.org/wiki/Accuracy_and_precision

User avatar
chris_mcmartin
Posts: 2433
Joined: June 8th, 2010, 11:13 pm
Location: Greater Houston TX Area
Contact:

Re: Deriving and Using Coordinates with Herp Data

Post by chris_mcmartin » June 25th, 2013, 6:47 am

Important to note--the H.E.R.P. Mobile app lists the accuracy of your GPS coordinates in METERS, not FEET...so if you're waiting for the accuracy to get down to 9, or 6, or 4, realize that translates to (approximately) 27, 18, or 12 FEET. I think there was some confusion earlier in the thread about this.

Post Reply