Posted 22 April 2010 - 01:44 PM
I've got a question that's been burning the back of my mind for some time now. My office is looking into purchasing a GPS unit with the intent of utilizing geographic data to be consumed by outdoor recreation enthusiasts. Although I had a somewhat firm understanding of this area 4 years ago, I'm a little rusty on developments that have occurred since. I wonder what units members of the geospatial community here at Cartotalk prefer?
#a. I have a tendency to be what I would like to call "an accuracy enthusiast" (my classmates at college use to tease me stating that you could "launch missiles with 99 percent accuracy" using the geographic data that I generated). However, this passion/obsession is not economically feasible, so I am looking for some guidance as what constitutes an acceptable level of positional error? This thread was most helpful, but didn't quite answer my question (for example, how do maps with multiple levels of zoom affect the "accuracy of the data?" Would it be possible to control error by digitizing at a specific scale?). Keep in mind that this is for recreational use, so I'm thinking that there is some leeway here...
#b. Although I know "GIS grade units" are much more accurate, the price might be somewhat prohibitive. We might have to employ individuals to collect data that do not have access to the funds necessary for more powerful units.
Thus, with these constraints in mind, what do you folks see as being the "most accuracy for your buck" so to speak?
As always, input is greatly appreciated!
Posted 22 April 2010 - 01:57 PM
Different scales of the same map don't effect the relative data accuracy in anyway unless you generalize as you scale up. The best practice here would be to determine the best scale you'd be using and digitize to that scale, generalizing from there as you rescale to smaller scales (larger areas). Don't include small insets in this calculation since they often cover very small areas and can be produced separately.
Something to consider is that a lot of your input may be able to come from other readily available sources versus gathering them yourself. Digitizing from air photos or USGS and Forest Service topographic maps may give you the accuracy you want without the need to invest in expensive equipment or a lot of field time... but I don't know the details of your project.
GIS Reference and Instruction Specialist, Stanford Geospatial Center.
Posted 22 April 2010 - 02:09 PM
In short all recreational grade GPS units are capable of the same accuracy generally speaking. Until you make a commitment to purchase a mapping/survey grade unit you will not see any difference. That being stated, it also depends on the type of data you are trying to acquire. Currently survey grade units will not collect data in harsh environments no matter how much they cost. So why spend the money to acquire the same level of accuracy that could be attained with a consumer grade unit? There are many options available for consumer grade units however you need to make sure that the unit will collect the features in an efficient manner and is capable of transfer the data into a usable format. Please be more specific about the project.
One final word about accuracy, I have had many conversations about this topic and it is always relative to the project at hand. By this I mean if your producing a recreational product using survey grade data if available is fine but I would not require for the project unless you have a lot of time and money. In short don’t get hung-up on accuracy. Accuracy is important but keep it in perspective. Read about the national mapping standards and strive to meet those specs and you will be fine. Hope this helps.
Posted 22 April 2010 - 02:33 PM
But, if you are interested in having people locate hard to find items (geocaching I presume) in hard to get to places where the scale is going to be considerably larger you may need to look into high accuracy data collection.
If you are looking to purchase GPS data logging equipment and are concerned with accuracy, you may need to step up to something a little pricier than the Garmin units you can buy from Walmart (FYI these work great for 99% of what most people will need).
My company uses survey grade Trimble GPS equipment to collect data for pipelines wellsites and facilities in the Oil and Gas Industry. With said equipment + qualified geomatics technicians + proper pre-planning regarding best time of day/weather/etc. you can expect data quality in the 2-4cm range or about 1". We also use high-end Trimble mapping grade GPS which with the same planning and technicians can produce sub meter accuracy or about 3'.
The more accurate the data needed the more expensive and extensive the equipment becomes and the more traning is needed to use said equipment effectively.
Hope this helps a little.
Posted 22 April 2010 - 02:46 PM
GIS Reference and Instruction Specialist, Stanford Geospatial Center.
Posted 22 April 2010 - 07:31 PM
Yes and no. It depends on the unit and the software being used to perform the correction. I would lean in the direction of using a satellite-based augmentation system (SBAS)/WAAS for simplicity and cost. Most consumer grade units have this feature available and it provides a real-time solution in lieu of having to post-process your data against a base station. As we all know anything is possible but there is always a cost associated with it. Trimble provides a planning application for free that can be used to insure the SBAS corrections are available for the date, time and project area.
Posted 27 April 2010 - 01:10 PM
Edited by honeybee77, 27 April 2010 - 01:11 PM.
Posted 28 April 2010 - 12:10 PM
Check out Trimble's Mission Planning software;
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Posted 05 May 2010 - 05:47 PM
Regardless, the impression that I'm getting is that I shouldn't be TOO hung up on the accuracy issue. I've done some scale calculations and my homework suggests that the average amount of error contained in my receiver will be appropriate for the scale in question about 80% of the time (with the remaining 20% being very close to said cuttoffs). I just wanted to get an idea of what was acceptable in the industry for recreational grade spatial data as my previous professional experiences has been in engineering/utiliites/academia (different beasts indeed).
The most effective solution it sounds like is to make use of the WAAS network as well as the mission planning software suggested by Mr. Rattai in order to contain DOP at time of collection. This is not survey/municipal grade data here, but rather for the outdoor enthusiast. I think some "squishyness" in the accuracy is acceptable (which I have to be content with due to time/budget/resource limitations).
In particular, David and Canvas101, thanks for the input on clarifying some of how to get the best data out my receiver as possible. And Darren, I'm looking into that mission planning software as I type this.
I have another question, but I think it belongs in another thread...
Thanks again everyone!
Posted 05 May 2010 - 06:39 PM
One new development with regard to WAAS. One of the two satellites that provides the WAAS correction is down. The FAA is looking into a solution but it may be several weeks before a solution is available. What this means is if you are using the planning software that I mentioned you will need to account for PRN 135 not being available. I believe PRN 138 is the other space vehicle and it is good. I think it is important to understand what is going on because you may experience some issues in the field.
On a side note I have included the link to the National Mapping Standards for your review.
Posted 06 May 2010 - 03:30 AM
From the technical point-of-view, I'm not even sure that augmentation improves accuracy, though it does appear to improve precision.
Just recently I've been working with groups of volunteers recording the position of point-features in a landscape using recreational GPS receivers and plotting them. I'd suggest that it's probably best to not bother too much about things that you can't do much about, like GPS errors, and concentrate on eliminating the errors you can do something about; conversion errors, in particular.
Posted 06 May 2010 - 09:12 AM
One quick comment. The difference between an autonomous position and one that has been collected with a WAAS correction is considerable. Autonomous +-15 Meters / WAAS +- 1-3 Meters. As you mentioned there are limitations as I pointed out in a previous thread.
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