compare measuring wheels?

Discussion in 'General Industry Discussions' started by Jacey, Dec 14, 2011.

  1. Kiril

    Kiril LawnSite Fanatic
    Messages: 18,334

    With all due respect David, you don't know what you are talking about. Survey grade GPS units can reliably produce sub centimeter accuracy, if you have 10-20K to dump on that kind of setup. You will never get anything even close to that with a consumer grade unit. I have two high end consumer grade GPS units that vary in accuracy from 6 to 20+ feet ...and that knowledge comes from plotting collected coordinate data on a 6" HRO. Bottomline, consumer grade units are not that accurate.

    Also, just because a GPS unit reports speed to 0.1 units doesn't mean it is accurate, nor that your speedometer is inaccurate. Your vehicles speedometer will always be more accurate unless it is broken.

    There are numerous ways you can get accurate square footage without using a GPS unit. Just because you are not aware of them does not mean they don't exist.
  2. NO! They do not exist!

    I DO know what I'm talking about. I had to take a course in surveying on my way to a career in engineering for the old Generel Telephone Company of Ohio. I have several pieces of surveying equipment sitting in my garage that "walked away" with me when digital cell phone technology replaced the horse & buggy technology of the old rotary dial phone, eliminating my career.

    GPS units are doing the same thing in surveying.

    Like most other misinformation your post cantains an element of truth. Consumer grade GPS units do not measure in sub-centimeters (also known as millimeters). That's irrelevant when measuring a lawn for an estimate.

    Tools are only as good as the person operating them. The Garmin has a built in fail safe when running an area calculation. You push the button, walk around the yard, and when you return to precisely the same spot you started it will allow you to press the button again to make the area calculation.
    Here's the "fail safe". If you're not in the same footprints where you began, the calculation is wrong. If you've returned to exactly the same footprints, AND the GPS allows you to make the calculation, the calculation is correct!

    Here's the part where I expalin why you can't get area calculations anywhere else than a GPS.
    Surveys are made with straight lines. Because there is no way to calculate area with curving lines that have a varying radius. The GPS can measure a curve. Nothing else "on earth" can.
    It's true I'm splitting hairs here, because if you're measuring a bed for mulch who cares?

    The advantage the GPS gives you is thet most measurements contain so many changes of directions by the time you write it down it looks like a piece out of a jig-saw puzzle. Plus without a transit to get a precise measurement of the angles all your calculations will be wrong.

    GPS does all that calculating for you. It's so accurate I use it to calibrate my fertilizer spreader. I read the bag, and it says it covers 12,000 sq. ft. I measure the lawn and it says 36,000 sq. ft. Three bags, right? I adjust the spreaders' flow rate or my pace or how close I space the rows to get exactly 3 bags down. With todays prices of fertilizer I could save the $100 for the GPS in one season. Plus the lawns will have the "ideal" application.

    It's time to step into the present day technology. GPS aren't any more difficult to operate than a cell phone. And they're cheap as a decent measuring wheel which would give area calculations that wouldn't be any better than a good guess.

    I'll stake my name on it.
  3. Kiril

    Kiril LawnSite Fanatic
    Messages: 18,334

    Yes they do .... it is called calculus, geometry and trigonometry.

    What does this have to do with anything? You are pretending that consumer grade units are producing survey grade measurements .... they don't .... not even close.

    Ummm, my post contains no misinformation David .... and FYI, you are lucky if you hit 3 meter accuracy with consumer grade units .... more likely somewhere in the 4-15 meter range, as I have already pointed out. Now if that level of accuracy is OK with you, fine, but it may not be OK for others. Is there something wrong with providing some facts about consumer grade units here?

    Again David .... it is called calculus, geometry and trigonometry. Now why wouldn't an engineer know that? :dizzy:

    Even breaking it down to simple geometric shapes will yield a better estimate than the consumer GPS in most cases, particularly if you are using waypoints in a situation where tracks would be more appropriate. Beyond that, how exactly do you think GPS units calculate area? Ever heard of a polygon?

    I would never use a consumer grade unit to do this, and again, this is based on the observed and verified performance of two high end consumer grade units. You might be OK in an open field to get a rough estimate, but the variability of the readings can lead to differences of 100's of square feet, especially in urban environments. Now if that is the kind of accuracy you are comfortable with then by all means knock yourself out. I'm just spelling it out like it really is so people know what they are getting into.

    If people can't do basic geometry using a measuring wheel to get a fairly accurate estimate of area, then they need to learn.
  4. ReddensLawnCare

    ReddensLawnCare LawnSite Bronze Member
    Messages: 1,651

    If Davids thing works for him, great! If you dont like it krill, make you point and leave it alone. As for me, I use GoiLawn and I really like it, its not perfect, and if the property is very new or has had some major landscaping done, it is no good, but for established neighborhoods and home plots, it works for me. The past two years I have worked on getting my name out for doing fert and squirt, so it works good for me. I never use it for giving a mowing estimate simply because its best to walk the property.
  5. Hey, Hey! No need for name calling here!
    Krill being an ocean crustation that whales eat, but when used to describe someone it's derogatory, indicating a particularly low lifeform.
    Though I did get a chuckle when I read it.
  6. I Googled Giolawn and really? $460/year for something you can get for free off of Google earth?

    If you had a GPS in your pocket when you walked the lawns you'd HAVE the actual square footage.
    And like I tried to expalin before, if the info you get fron the satellite (the path around the yard will close) matches what you did on earth (you're back in the footprints where you started) the measurement is precisely accurate.
    I've checked it several times against property deeds.

    I've written enough easments on deeds to know there is no terminology to describe, nor is there any way to measure or reproduce on land an irregular curve. But GPS can.

    I think there's somebody who should take their measuring wheel and beat his two piece-of-crap GPS units to bits and go buy a Garmin e-trex vista. Unless he wants to keep on being wrong about everything.
  7. ReddensLawnCare

    ReddensLawnCare LawnSite Bronze Member
    Messages: 1,651

    Its not 460 a year and my bad on the spelling haha. It is 50 to get started and then 3 bucks for each measurement and hAs a lot more features then google earth. Try the free trial. The 460 is for proposal builder
    Posted via Mobile Device
  8. Kiril; My apogogies, you're right. Consumer grade GPS isn't that accurate.

    But I'm also right and here's the reason.

    Consumer GPS will give you coordinates within say 16-20 feet somewhere on the planet.

    If you're measuring the area of a lawn you're doing what surveyors call a closure. You take the length of the sides, and the angles and make your calculations. If the calculations bring you back to the original starting point the calculations are correct. Then you can figure the square footage with the angles and lengths from the irregular box you've created.

    The Garmin e-trex vista has an area calculater built in. It's rare in consumer GPS units. When you press "start" and walk the perimiter of the yard it creates a line on the screen's map where you've walked. When you get completely around the lawn the line meets up with it's self the option "calculate?" appears. Push the button and it gives you square footage.
    Now if the lines meet up, AND you're back in your starting footprints That means the satellite GPS measurement matches what just happened on earth, and if they match they're correct to a fraction of a square foot.

    BUT! what about the 20' error? It doesn't matter if you've run your closure 20' to the west, or north or wherever you're supposed to be. If you've got as closure, you've GOT a closure. It's accurate! I've checked it too many times against deeds etc. not to believe it.

    If you've had interference from tall buildings or overhanging trees or something, either your lines don't meet up or you're not back in the same footprints where you started. Do it again. Or get out your measuring wheel and do it by hand.

    Sorry it took me two days to figure this out.

  9. Kiril

    Kiril LawnSite Fanatic
    Messages: 18,334

    David, as I said before. I have verified the accuracy of two high end consumer grade units on 6" HRO. In fact, at the time I bought them they were considered the most accurate consumer hand held units on the market.

    Beyond that, the majority of the points I collect are an average of 100+ position readings, which should make them considerably more accurate than a single collected point (i.e. your walking), and yet they aren't. Almost every point I collect with consumer grade units need to be adjusted using physical measurements when I project them, which essentially makes the consumer GPS worthless. They bottom line is consumer grade units are not accurate, and again, you are lucky to get accuracy to 16-20 feet, especially in urban environments.

    If you have a couple of points that end up 20+ feet outside your area, some points that are 10 feet inside your area, etc ..... what does that do to the calculation? It screws it up. It doesn't matter if you have "closure" as long as your points don't cross over themselves. You are making the incorrect assumption here that your error is consistently the same .... IT IS NOT.

    In an open area (i.e. open sky, no buildings, trees, etc... ) you may get acceptable results, assuming the area is large enough to absorb any errors. In the case of urban landscapes, the areas aren't large and you have considerable problems with multipath rejection due to a variety of reasons.

    Now if you can live with the inaccuracy that is fine, however I wouldn't use it for anything other than determining bulk material requirements.

  10. Isn't that what we're after? It's not a survey of the property and you can experience interferrence. Other than that it's a better tool than wheeling it and doing a page of calculations.

    I seem to have more of a problem explaining your experiences and abilities than you have explaining what I'm capable of.

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