Contour Intervals - Can someone explain this to me

I sometimes would have people request 2 foot contours instead of 1 foot contours.
I'd explain that 2 foot would cost more, since I had to draw the 1 foot contours, then erase every other one.;)

But 20 meter contours? What are you topo'ing, the Himalayas?

Ridiculous that he thinks cost will be less with a lower contour interval. This guy has no idea what he needs and probably shouldn't be commissioning the survey.

Of course the amount of work involved does depend on the contour interval. The planimetrics is the same effort (depends on scale), but you don't need anywhere near as many ground shots for 20 m versus 1 m, and you can skip a lot of the "minor" topo features.

But that is a huge jump from a 1 m to a 20 m interval, I agree he probably doesn't understand. The whole eastern US (and much of the rest of the country except in the mountains) was mapped at 1 20 foot contour interval, so he wants 3 times less? Is he having a site in the rockies mapped???

There would be a significant reduction in the fee for me, but I live near sea level and I have worked on very few sites that would have a 20 Meter interval!

Same thing 'round here. I would have one contour on the map.....maybe.

It doesn't sound like the potential client knows what he is asking for. Is he aware what that contour interval would look like on the plan?

John Hamilton, post: 381079, member: 640 wrote: Of course the amount of work involved does depend on the contour interval.

Most of the sites I work on I would still need the same amount of shots given the amount of break lines usually involved, so final plotted contour interval would not make much difference. Now if it was just an open area with no break lines, streams, roads etc. then maybe there would be less shots but that would be the exception.

I wonder if they meant "20 cm"? Which of course would increase your fee.

Perhaps he wanted shots on 20m grid.

Perhaps he thinks you were going to take spot elevations every one meter horizontally on a grid.

Sounds like a typo to me. Is that supposed to be 2m contours?

might be easier just to follow the 20m contour line, can't be many of them.

Your local GIS probably has data better than that. Download it and lay it over your planimetrics. In fact, perhaps this client just wants you to acquire the GIS level stuff.

Some architects and engineers think like that. You are thinking 1m (vertical releif) He wants shots on a 20m grid. Griding is the old way of doing things. Last engineer wanted a 20' grid from me, I said no problem. Went would topoed the area randomly, got the CBs, dips valleys high spots, and enough data to topo the site correctly. Created a tin, and then labeled the site on a 20' grid. Added a note that elevations were interpolated from actual elevation shots, and walked away, everyone was happy.

A Harris, post: 381093, member: 81 wrote: Perhaps he wanted shots on 20m grid.

I bet this is it. Even a meter interval would be big around here. We typically do 2 foot. It generally will cost more to do a higher interval, since it requires more information.

Of course contour intervals are indicative of precision, but, as I see it, it doesn't effect the surveyor's field work all that much. You still need to follow all the break lines, and hit angle points of the break. And I imagine that they field guys would want to get those extra shots on plain ground, that would be able to push it up to a higher standard since they are walking every inch of the site anyway. Also, it doesn't eliminate any improvement shots. Finally, if they did say they wanted a better interval, you would have the field work done anyway.

It makes a much greater difference in an aerial job because they would make a higher flight and would require less panel points and a lot less number of passes over the site.

My reasoning is even if he wanted 20m contour interval (yes I confirmed it with him) the field guys using RTK/PPK would traverse the same area - trees,breaklines,waterways etc.
The only item that would be lowered is the cost of the ink for not plotting contour lines 2-19.
I remember when my guys surveyed an prairie. It was relatively flat so they got data at 50m grid intervals. When we processed the contours in the office, it did not look right. It was going all over the place. Had them go back to add more grid line data on the ground (~10m-20m). This time contour lines came out better looking.

I was taught that contours had to be accurate to within 1/2 of the contour interval.
to meet this requirement - larger intervals = less shots

TickMagnet, post: 381232, member: 4378 wrote: I was taught that contours had to be accurate to within 1/2 of the contour interval.
to meet this requirement - larger intervals = less shots

National Map Standard accuracy requirements said, 90% need to be within 1/2 contour interval

This is a 1947 standard, there must be a new one by now? Jp

Working on memory here but as I recall the standard says all points sampled are within 1 contour interval, and 90% of the points sampled are within 1/2 the contour interval. So do you really have to shoot every little break line and every little jog in every break line to guarantee that every point on the map is within 10m vertically? Of course not. You can ignore that break where it changes from 7% to 7.5% slope. You can ignore that little 1' ditch alongside the dirt road. You need the MAJOR breaks, and then only at widely spaced intervals. In many cases you could just shoot the 4 corners of the parcel and generate 20m contours to that standard, so in most cases it would certainly lower the cost. The guy is trying to tell you that he just needs a rough topo, maybe for feasibility study or something? Telling him that a rough topo costs the same as a really good topo probably doesn't make him think he's getting what he's paying for.