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# Topo contours clumped every 1 meter

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02-11-2021 08:12 AM
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Occasional Contributor

Hi,

I'm using ArcMap 10.1 and I'm trying to make contour lines from a DEM. I've made 1m contour lines which are fine, but I want contour lines every 10cm. I was able to make them, but they are clumped together every meter. So the 8.9m contour is half way between the 9m and 8m contour and the 8.8m-8.1 are squeezed between the 8.9m and 8m lines. If that doesn't make sense I've attached a picture showing this.

How can I make it so they are evenly spread out? The data I have isn't this precise so I know this is just an estimation, but it still seems strange that the lines are clumping together. Any thoughts?

Thanks,
David

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Occasional Contributor

@SteveLynchexplained it to me with this simple image. Makes a lot more sense now.

12 Replies
MVP Frequent Contributor

What tool and inputs did you use? What's the purpose of 10cm contours from a DEM which isn't as accurate as that?

MVP Esteemed Contributor

What was the raster cell size?  That will have an impact on the resultant contours

... sort of retired...
Occasional Contributor

I used the contour spatial analysis tool (maybe I should have put this in a different group, whoops). The raster cell size is 10 by 10 meters, I believe.

I'd like 10cm contours because this is the best available data I to evaluate sea level rise (even if estimations will be rough) I need 10cm contours. The contours I'm getting now though are just strange and I don't get why they are coming out like this.

MVP Esteemed Contributor

We are trying to get a sense of whether those contours that you show are all within one 10 meter cell.

This is far too coarse for getting 10 cm contours.  See the figures in this link.

How Contouring works—ArcGIS Pro | Documentation

I would look for another source of data if this is for a small area, perhaps 1 m resolution or better (eg lidar)

... sort of retired...
Occasional Contributor

I just zoomed in on the contours to show my problem, I'm covering an area of about 450km2 and 1500km of coastline. They are developing new data, but it won't come out until November 2021 at the earliest. Maybe I'll just have to wait.

I'll check the link you provided. Thank you for all the feedback.

Frequent Contributor

At any precision, each contour represents a closed polygon that represents where the elevation is above or below the break value, such as 10m. 10m contours draw completely different shapes than 100m contours, with many more bends  than 100m contours. They should not be evenly distributed. They may even cross the 100m contour lines because they are working from more information.Brown is 20m line; greet is detail

Occasional Contributor

Thanks for your response. What you're saying makes me even more perplexed at why the contour lines I'm making coming in so weird.

Maybe this picture shows it better. Every group of contour lines is 1m and you can see that in these groups are each of the .1m contours. It almost makes it seem like the land is terraced, but it's not..

MVP Frequent Contributor

You may well have this issue if the DEM raster is integer type Problem: Contour lines look blocky, appearing to trace raster cell boundaries (esri.com)

As to the other issue, @DanPatterson @DanaNolan 's replies outline why this would never work, i.e. expecting to create contour intervals far less than the base resolution of the input DEM.  Why it creates the contours in such a fashion around the base contour then stops is likely a mystery hidden within the tool.

Either way it's not relevant, as it's just a case of Garbage In Garbage Out.

Occasional Contributor

Thanks, I looked at the page you linked to and tried the solutions. They did smooth out the contour lines, but they still clump together for some reason...maybe you're right and it's just the data.