The geographic coordinates of a geo-located model coincide with SketchUp’s world axes origin [0,0,0]
The elevation of GE Terrain at those geographic coordinates also coincides with SketchUp’s origin [0,0,0]
Given you know the real world elevation of the model’s geographic coordinates…
Using TIG’s plugin, you can assign a real world reference height to the Origin, enabling you to measure other points in the model in reference to that real world datum.
Minor point @colin…a mile equals 5280 feet so the highest elevation quoted by @huckrorick is roughly a .25 mile distance and it’s even less when the initial Z coordinate of 319’ is subtracted. Though your point is well made the five mile reference is exaggerated.
I don’t think I understand. If the world axes origin for SketchUp model and GE Terrain are the same, shouldn’t the elevations show the same in my model and on Google Earth?
I am using this information mainly to get differential elevations (for a gravity driven water system). I was assuming a could subtract the lower elevation from the higher and therefore get the elevation difference and gravity fed water pressure. That differential is more or less the same when measured on Google Earth and measured in my SketchUp model so for my immediate purpose the fact that the elevations are different in Google Earth and SketchUp doesn’t affect my current problem (only the difference matters, not the absolute elevation). But still, the discrepancy puzzled and bothered me. Why are they different?
Su and Google Earth both use the WGS 84 datum for the earth. The zero elevation is a mean sea level and the lat, long is based on a number of spherical harmonics to curve fit the Earth. See this for more info North American Datum - Wikipedia. The zero meridian no longer goes through Greenwich for this datum. Many locales use their own local datum for elevation like the Ordinance Survey in England and others for reasons you can search if interested. ( For example boats going under bridges, plate shifts, weather. etc.)
The elevation info in Google Earth is determined from satellite measurements from a number of different sources and of course will not all result in the exact same values just because or system design, weather, orbit accuracy ,etc. The GE data will usually show the source data on the image and I would guess Google Earth does not contract for the highest accuracy data just because of cost. Google Earth also has an terrain exaggeration(.01 to 3) used for display.
You as a user must under stand the accuracy of the data used and make sure it meets your requirements. I have not seen any error analysis but it must exist.
Just some thoughts for you.
Personally, for the task at hand I wouldn’t place too much faith in heights derived from GE.
Field measurements will tell the true story, say, a few shots with a total station.
Then again, if distances aren’t too great, one could measure actual pressure developed on site, using nothing more elaborate than an ordinary garden hose, filled with water, fitted with a gauge at the bottom.
I ran in to a puzzling result in making measurements of water pressure. I have a pipe that runs from one of the relevant locations to a lower location. I put a pressure gauge on the pipe at the bottom. It measured 15 PSI. This didn’t make sense in terms of the elevation differential that I knew existed approximately. I opened a valve in the pipe and ran some water and shut the valve. The pressure then measured 46 PSI. This was close to the expected pressure. I drained the pipe and then filled it again. I got 15 PSI. this time nothing I did changed the pressure to the expected and previously measured 46 PSI. How could this happen?
Thanks for thinking about this. It really puzzles me. The gauge is good. I have used it on other locations with good results.
It is puzzling to me how air in the line could affect the pressure. Wouldn’t it be compressed and pass that on down the line? I guess if there were a lot of air then It doesn’t weigh what water does so it wouldn’t contribute to the pressure, but I’m running water through the line (coming out where the gauge is, before attaching the gauge) so it doesn’t seem like there could be a lot of air.
Laws of physics are well established in this area so one needs to question what you are doing in the test to account for result since you have establish measurement device is ok.
Water is non compressible while air is compressible. When charging an empty flow restricted pipe the air vol. will be compressed until its pressure equalizes and to first order follows the universal gas law pv=nRt. Very small vol. release can result in large pressure changes. I would suggest you have pressure gauges at both ends connected via t’s such you can bleed and will give you ref. at the head input and measure points in the test system. Homes will have pressure variations as supplied by your utility and some homes have pressure regulators. Spec is probably in 60 psi g range and low in the 30 range. What you measure can be an function of neighbor’s use ? If you have regulator in system make sure its ok?
Sorry for typing errors just had split put on thumb this PM.
The system is gravity fed (rural, private). There is no one else on the system. It is a water tank at the top end with a 1 ¼” PVC line from the tank at the top to the bottom end where I am measuring pressure. No pressure regulator. There is no pressure at the top end except for the depth of water in the tank (about 4’). The gauge is placed on a hose bib at the end of the 1 ¼” pipe. I measured 46 PSI on several occasions. Then did a test where it showed 15 PSI. After opening and closing a valve it jumped to 46 PSI. After draining the pipe and then turning water back on (at the top) I measured 15 PSI again. Opened and closed the valve, but the pressure did not go up to 46 PSI as it had previously.
FYI there RE many pressure calculators on the net you can use. If I could get thru my thick head exactly what you set up is I would take crack at what you should expect=> like I said physics does not lie so some thing is wrong in your approach and I am getting lost in top vs bottom, the pipe length and exactly how you are making measurement. You should be able to make a quick SU diagram. to let us all know. Typically pressure gauge accuracy is specified at full scale so a 100 psi g one at 10% it could have 10 psi-g error at the 15 psi-g level. To make accurate measurement you want to measure close to full scale.
There is over 875’ horizontally between the upper tank and the hose bib where I attached the gauge (it is at a lower tank which is shown on the attached SketchUp drawing) so I can’t easily determine the elevation with a survey and I don’t have sophisticated survey equipment. The most accurate measure I have of the elevation difference is the measured pressure which has been measured at various times in the past. The consistently measured pressure is about 46 PSI. It will vary a bit depending on the level of water in the upper tank. The elevation difference measured on Google Earth is 95’. Google Earth is not very accurate. 95’ corresponds to a pressure of 41 PSI which is reasonably close considering the uncertainties of Google Earth.
The big question is: What mechanism could account for measuring 15 PSI, and for that measurement being different at different times (sometimes 46 PSI) when nothing is physically different in the pipes and tanks? And for it changing from 15 to 46, again with nothing physically different in the pipes?