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#1
The Doomed Mapper

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Hello out there everyone!

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?

Some guidelines:

#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!

#2
David Medeiros

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The overall accuracy you'll need to strive for should be driven by the scale of the map which will be at least partially determined by its intended use and audience. There's no point in sub meter accuracy if your doing a driving tour map for instance. A hiking map may require a bit more detail, though exact trail placement down to the inch is not required.

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.

 

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#3
canvas101

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Hello Doomed Mapper,

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.

Regards

#4
Darren Rattai

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Are you intending to provide your clients with a map or simply X & Y coordinates to said features? This will affect what you need in terms of accuracy. If you are simply producing a map for them, then as stated already, accuracy may not be a huge concern, depending again on scale, you could get away with secondary sources of data.

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.

Cheers,
Darren

#5
David Medeiros

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and correct me if I'm wrong but you can use differential correction on data from consumer level "mapping" units?

GIS Reference and Instruction Specialist, Stanford Geospatial Center.

 

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#6
canvas101

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Hello David Medeiros,

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.

Regards

#7
honeybee77

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I agree with what others are suggesting. Even with geocaching, keep in mind, most folks gather locations using recreational GPS units by amatures. In other words, even when seeking a small, hidden item in the big world, geocachers can get away with using lower accuracy GPS receivers. If you are talking about locating some kind of POI for your town, I think you'll be fine with recreational GPS. Even if you're off by a few feet, hopefully a person is smart enough to look at his/her surroundings and see the intended feature. You just need to get them close. As a geocacher myself, one suggestion that's often given is to go back to the location several times on differerent days and average the coordinates. Some days the satellites are lined up perfectly, but other days not and hopefully this averages out those bad days.

Edited by honeybee77, 27 April 2010 - 01:11 PM.


#8
Darren Rattai

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Many GPS equipment companies offer free Mission Planning software. Simply enter your location and which day you are planning on going and it will provide the best time that day in regards to satellite coverage and lowest DOP (dilution of precision) value.

Check out Trimble's Mission Planning software;
Visit My Website


Cheers,
Darren

#9
The Doomed Mapper

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Wow, everyone, I just wanted to take some time to thank you all for the advice presented here in this thread. Please forgive me if the time to respond to your comments has taken me a few weeks; being buried under work and a drive for perfectionism in my online dealings can be something of a dual curse at times.

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... :lol:

Thanks again everyone!

#10
canvas101

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Hello Doomed Mapper,

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.

http://rockyweb.cr.u...mas/NMAS647.PDF

Regards

#11
Nick H

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I'm not so sure about the benefits of WAAS and EGNOS when using recreational GPS receivers. The satellites that broadcast the correctional data are in geostationary orbit over the equator, which means you have to have a clear view in a roughly southerly direction to receive the data. You don't have this in many places, in woodland for example.

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.

Regards, N.
Caversham, Reading, England.

#12
canvas101

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Hello Nick H,

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.

Regards




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