Elevation Calculations

I've been using site levels and lasers for years now. I'm fairly well versed with them. However, I am currently working on a project that has me stumped. I have 4 catch basins with 24" storm drain in a line. I am trying to find the elevation of each catch basin. My benchmark is set at 157.91. If I set the benchmark on my laser rod and it's set at 3.25(for example), if my 1st basin shoots a 4.50 on my rod, I calculate the difference of 1.25. I then subtract 1.25 from 157.91 and get the elevation of 156.66 for my 1st basin. My question is, if my 2nd basin shoots a 5.50 on my rod, do I subtract the difference between 4.50 - 5.50(1.0)from my original benchmark, or do I subtract it from my 1st box elevation of 156.66? I really hope this makes sense as it's kind of hard to explain. Anybody? Thanks a bunch in advance.

Your bench mark is your bench mark, that's what you add/subtract from, you can have an infinite amount of points on the plane, but they don't effect your bench mark.

That's exactly what I thought. Since the benchmark never changes, that's what I would always refer to. I greatly appreciate your reply!

Same result, but you might consider calculating your HI (height of instrument) and working off of that figure, rather than keeping track of one rod reading, and then working off of that rod reading. In your example: BM=157.91, Rod Reading @ bench mark = 3.25, Height of instrument = 157.91 + 3.25, or 161.16. Use this and then subtract the designed height of the basin to figure what the rod reading should be if at grade. If your first basin is designed to be 156.66, then subtract that from the HI 161.16-156.66=4.5 rod reading. For the next basin subtract the design height from the HI again. As I mentioned, you will get the same result but it should cut down on mistakes made from trying to remember rod readings. If you need to move your laser, then shoot the bench mark again and re-figure the HI. This method sometimes seems slower than just comparing the previous rod shot, but it has helped me to save time by not making (as many) mistakes. I also have finally got in the habit of writing all these things down rather than trying to keep them in my head, which also has cut down on mistakes. The old noggin ain't what it used to be!

I get white primed 2x2s from the lumber yard. When at the bench, I mark the 2x2 at the laser bracket with a 0, or, the benchmark number. Off I go to the next location and move the laser receiver bracket to the new location and then literally measure. A tape measure with inches in tenths facilitates this. ~ Same as the other way, just less math.

Agree on writing down. 3x5 cards help this task.

Oxbow said: Same result, but you might consider calculating your HI (height of instrument) and working off of that figure, rather than keeping track of one rod reading, and then working off of that rod reading. In your example: BM=157.91, Rod Reading @ bench mark = 3.25, Height of instrument = 157.91 + 3.25, or 161.16. Use this and then subtract the designed height of the basin to figure what the rod reading should be if at grade. If your first basin is designed to be 156.66, then subtract that from the HI 161.16-156.66=4.5 rod reading. For the next basin subtract the design height from the HI again. As I mentioned, you will get the same result but it should cut down on mistakes made from trying to remember rod readings. If you need to move your laser, then shoot the bench mark again and re-figure the HI. This method sometimes seems slower than just comparing the previous rod shot, but it has helped me to save time by not making (as many) mistakes. I also have finally got in the habit of writing all these things down rather than trying to keep them in my head, which also has cut down on mistakes. The old noggin ain't what it used to be! Click to expand... This is a big help, too. I've never done it this way, but it definitely makes sense and I understand it. I'll try it tomorrow. I wear shirts with pockets, where I keep a pen and paper. I have to write all of these numbers down or it just becomes too much. Thanks for your help!

oceanobob said: I get white primed 2x2s from the lumber yard. When at the bench, I mark the 2x2 at the laser bracket with a 0, or, the benchmark number. Off I go to the next location and move the laser receiver bracket to the new location and then literally measure. A tape measure with inches in tenths facilitates this. ~ Same as the other way, just less math.

Agree on writing down. 3x5 cards help this task. Click to expand... I use a tape measure constantly. It's way too easy to screw up the tenths/inches conversion. Thanks for the response!

I've worked hard with my guys on using elevations with a Lenker rod. It cuts down on the mistakes by taking all the math out of it as the guys just have to match the number on the plans with the number on the rod. They do cost a little bit, but if taken care of they last and the time saved pays off.

We do the same when setting storm structures, pipe and grade elevations. I stress to my guys to always reference the bench mark, not the last structure set in order to cut down on mistakes. For example the day before we set two manholes and ran 100' of pipe. At the start of the day set up the laser, reference the bench mark and check the last structure set. If there is a discrepancy re-check your rod on the BM and find out what's wrong. It helps to cut down on errors. I use this an orange field book like this - http://www.engineersupply.com/elan-...2j19JZRKWPpw4o911fkZ1r6b4MljmPTGw3hoCoXfw_wcB Each day of pipe laying is recorded in the book with date and job. All the calculations for the day are written down in the book. I like the Lenker Rod, going to check into that. Looks like a simple way to reduce math errors and speed production.

CM1995 said: We do the same when setting storm structures, pipe and grade elevations. I stress to my guys to always reference the bench mark, not the last structure set in order to cut down on mistakes. For example the day before we set two manholes and ran 100' of pipe. At the start of the day set up the laser, reference the bench mark and check the last structure set. If there is a discrepancy re-check your rod on the BM and find out what's wrong. It helps to cut down on errors. Click to expand... I agree with checking the benchmark against last structure at the beginning of the day, but if you're dealing with structures such as manholes, once it's set it's essentially a benchmark, unlike floating grade such as a building pad or excavation. Of course, this all depends on how competent your guys are with math.

Lazer head with slope capability helps to keep calculations to a minimum

Shimmy1 said: I agree with checking the benchmark against last structure at the beginning of the day, but if you're dealing with structures such as manholes, once it's set it's essentially a benchmark, unlike floating grade such as a building pad or excavation. Of course, this all depends on how competent your guys are with math. Click to expand... Agreed. If your last structure was out a little sometimes you can adjust the next one depending on the flow line to get back to grade. Don't ask me how I know this...

CM1995 said: Agreed. If your last structure was out a little sometimes you can adjust the next one depending on the flow line to get back to grade. Don't ask me how I know this... Click to expand... That's funny you mention that........I was going to ask you about "oops" situations, because they happen to the best of us. I've did it both ways, put just a tad less slope for sometimes a few runs to get back to zero, or just leave it alone if it's not going to matter anyway.

Shimmy1 said: That's funny you mention that........I was going to ask you about "oops" situations, because they happen to the best of us. I've did it both ways, put just a tad less slope for sometimes a few runs to get back to zero, or just leave it alone if it's not going to matter anyway. Click to expand... It happens to the best of us. When it does happen and it will, if the water still runs downhill that's all that really matters.

When using a laser or instrument remember to have to checked. I have had quite a few that were off,had to redo pipe line or redo slab on grade.

Use a smart lenker rod. Just set bench to zero then red numbers for cuts. Blue numbers for fills. Or you can get a elevation and use zero on it. Less figuring to do.

Hoppsxc140 said: I've worked hard with my guys on using elevations with a Lenker rod. It cuts down on the mistakes by taking all the math out of it as the guys just have to match the number on the plans with the number on the rod. They do cost a little bit, but if taken care of they last and the time saved pays off. Click to expand... I checked out that Lenker Rod on You Tube. Don't know where I have been, but gotta get me one of those. Great tip!

This is a good thread. Today I was down at the street end of the job, and the hub I used to set the big grease interceptor was still there. When I did the job I took a picture of it and the lath, and also of my notes with the math, back in December. I just saw that structure again for the first time, I uncovered one of the castings sitting on top of it

Yes as mentioned a lenker rod is the way to go, if your bm is 157.91 then dial your rod to 7.91 then shoot your catch basins and you are reading direct elevation now you got the proposed elevations for every hole and can adjust accordingly so the one was 156.66 you would just shoot 6.66 anything either side high or low is your adjustment

CM1995 said: We do the same when setting storm structures, pipe and grade elevations. I stress to my guys to always reference the bench mark, not the last structure set in order to cut down on mistakes. For example the day before we set two manholes and ran 100' of pipe. At the start of the day set up the laser, reference the bench mark and check the last structure set. If there is a discrepancy re-check your rod on the BM and find out what's wrong. It helps to cut down on errors. I use this an orange field book like this - http://www.engineersupply.com/elan-...2j19JZRKWPpw4o911fkZ1r6b4MljmPTGw3hoCoXfw_wcB Each day of pipe laying is recorded in the book with date and job. All the calculations for the day are written down in the book. I like the Lenker Rod, going to check into that. Looks like a simple way to reduce math errors and speed production. Click to expand... Recording calcs for the day is great advice, I used to just scribble my math on some scrap paper or on the plan set, usually it would end up lost or just be some gibberish numbers on the plan. I made a Google sheet that auto calcs my slope % for each run. I just enter my starting and stopping elevations and run lenthgs and it spits out slope, everything is auto saved, I just make a new sheet for each job/task. Takes some of the guesswork out of it and gives a good record of what was installed. I always Calc my own slopes, I've caught many errors on plans, or had to adjust slopes due to how the engineer calced them. Example being some engineers draw pipe runs center of structure to center of structure, but really your actual pipe run is probably 4' less than that depending on what you're doing. Low slope stuff this isn't a big deal, but if you're running 15% (can be common here) you can end up .6' off on elevation. Good practice to always Calc your own slopes and double check