<html><head></head><body><div style="font-family: Verdana;font-size: 12.0px;"><div>Hi J.O. Williams</div>
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<div>thank you very much for your ideas and detailed explanation. I will try them out and let you know how it works.</div>
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<div>Eva</div>
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<div style="margin:0 0 10px 0;"><b>Gesendet:</b> Freitag, 06. November 2020 um 06:52 Uhr<br/>
<b>Von:</b> "J.O.Williams" <j.o.williams.jow@gmail.com><br/>
<b>An:</b> fantypus@web.de<br/>
<b>Betreff:</b> Re: [Qgis-user] How to get the thickness of a geol. layer from elevation isoline features</div>
<div name="quoted-content">Eva<br/>
<br/>
I only saw the end of the third reply. I added my thoughts to it.<br/>
<br/>
I hope it helps.<br/>
<br/>
J.O. Williams<br/>
<br/>
<br/>
<br/>
><br/>
> I have the following task:<br/>
><br/>
> I need to create a map with contour lines showing the thickness of<br/>
> one geological layer/horizon (actually not the true thickness, but<br/>
> the vertical extent is required, so if you drill down at one point,<br/>
> what is the distance between the upper and lower edge of the horizon).<br/>
><br/>
> The only data I have are two line features (shape files) of isolines<br/>
> with elevation values (in m NHN) of each the upper edge and lower<br/>
> edge of the horizon.<br/>
<br/>
This implies you only have contour (isolines) of the top and bottom of<br/>
the formation. Turn the isolines into points with a new attributes of<br/>
x, y and of elevation or use a 3-d shapefile point If the isolines x and<br/>
y values are in Lat-Long degrees, turn them into meters.<br/>
<br/>
If the isolines are steeply dipping, consider using Universal kriging.<br/>
<br/>
> My approach would be something like this, but I can’t figure out how<br/>
> exactly to do it:<br/>
> • Create a raster from each line feature (did that, but only the<br/>
> lines are rastered, so would need a polygon to get a plane? How do<br/>
> that? How can I include the elevation values of the lines into the<br/>
> polygon?)<br/>
<br/>
A problem with contour lines, is that there are lots of values along the<br/>
elevation contours. You might need to remove every other value. The<br/>
search routines of kriging follow the contour lines and don't find<br/>
values between each contour line elevation.<br/>
<br/>
>From the x,y and elevation values of the contour just run a inverse<br/>
distance (ID) or Ordinary Kriging (OK) of the contour lines. I would<br/>
suggest OK. You didn't mention if you needed the quality of estimation<br/>
that OK can also give you, so just running kriging run using a linear <br/>
variogram with a sill of 1.0 and a range of about 1/3 of the distance<br/>
across you area of of interest.<br/>
<br/>
The only reason to calculate a variogram is to calculate the estimation<br/>
variance. If you don't need the estimation variance, a variogram that<br/>
doesn't have a sill and keeps increasing will work just fine. Linear<br/>
variograms are cheap to calculate. If you don't have a linear<br/>
variogram, the first 2/3 of the spherical variogram is linear.<br/>
<br/>
Don't use nugget value. Nugget values are usually some sort of<br/>
measurement error, such as the Geologist picking the wrong value of the<br/>
top and bottom elevation of the formation.<br/>
<br/>
However, with the holes you have in the data and the faults you will<br/>
have to constrain the input data. Do not let the data search cross the<br/>
faults lines. When the kriging takes place, you want any trends in<br/>
elevation to continue until up or down until you hit a fault. Likewise<br/>
with other areas between the faults, they too will have to be kriged<br/>
separately so the data points outside the area don't influence the<br/>
calculations. Lots of clipping will be necessary.<br/>
<br/>
When you are done you should have two raster surfaces that have<br/>
elevation calculations that show each of the sub faulted areas. There<br/>
will be ugly elevation changes between the areas. Hide the ugliness<br/>
with very broad lines!<br/>
<br/>
> • Interpolate the elevation values in between the isolines to receive<br/>
> a plane of elevation values (in my opinion would need a polygon to<br/>
> get a plane, but for interpolation input Points are needed? And which<br/>
> interpolation method? Also, some of the isolines are offset by<br/>
> faults, so need sharp edges in these areas, how to integrate that in<br/>
> the interpolation?)<br/>
<br/>
The contour lines for each of these subareas will have to have an area<br/>
attribute added. The raster points will have to have a attribute value<br/>
that identifies each area. When kriging just use the rasters from each<br/>
area at a time and the elevation contour lines for each sub area.<br/>
<br/>
Each subarea will have to be done separately and checked against the<br/>
original contour lines.<br/>
<br/>
> • Then calculate the vertical distance at each point in the area<br/>
> between the two planes to get the thickness (how, which tool does<br/>
> that?)<br/>
> • Create a raster showing the thickness throughout the area in colour<br/>
> range<br/>
> • Add contour lines showing the thickness of the horizon<br/>
<br/>
Be sure you enter known contour line values during contouring. It is<br/>
very easy to get negative elevations. Negative elevations are VERY<br/>
embarrassing on the final The one attached example has a zero elevation<br/>
line in the north-west edge. You can draw a zero line, if you need, by<br/>
drawing one contour line at an elevation of say 0.1 meters but labeling<br/>
the line as 1 meter.<br/>
<br/>
If raster areas go negative, set the raster area to zero.<br/>
<br/>
Thick contour lines are your friend and cover up possible problems with<br/>
the data (and your presentation).<br/>
<br/>
><br/>
> Maybe there is even a better and probably much simpler way to do<br/>
> this. However, I am only a beginner in QGIS, python and statistics.<br/>
> So any help and easy to understand tips will be very much<br/>
> appreciated. I am using QGIS 3.10.11 by the way.<br/>
><br/>
> Best regards,<br/>
><br/>
> Eva<br/>
><br/>
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