topic Re: Contour line issues in ArcGIS Mapping and Charting Questions

Contour line issues

OvidiuFrantescu — Sat, 21 Jan 2017 14:00:43 GMT

Hi all,

I am working with a raster file (.tif) that I have georeferenced, and now I would like to extract the contour lines of of it.

After using the contour tool I am left with multiple lines representing a single contour line from the raster.

Basically the tool does not recognize only the center-line of the contour line from the raster to generate an unique line (see attached image).

How can I condense these lines, or generate a single (center-line) one.

Thank you,

Ovidiu

Re: Contour line issues

DanPatterson_Retired — Sat, 21 Jan 2017 14:43:03 GMT

what coordinate system is the data in? not the dataframe but the actual data?

what contour interval did you use?

What is the cell size?

I suspect you are contouring at too fine an interval relative to your cell size and the range of values in the raster

Re: Contour line issues

OvidiuFrantescu — Sat, 21 Jan 2017 15:02:49 GMT

Hi Dan,

The data is in WGS 1984 Web Mercator.

In the contour tool I selected the interval to be 1, because the original contours are at 1 ft. intervals.

The cell size is X/Y 0.013888889, 0.013888889.

Now that you mention these, I am thinking that I should do a raster re-sampling to increase the cell size.

Any advice is greatly appreciated.

Ovidiu

Re: Contour line issues

DanPatterson_Retired — Sat, 21 Jan 2017 15:31:51 GMT

those cell sizes look like decimal degrees and I suspect the data may be in GCS WGS 1984 ... which is unprojected data

Re: Contour line issues

SteveLynch — Sat, 21 Jan 2017 16:48:53 GMT

what is the min/max of the input raster and how many columns/rows does it have?

Re: Contour line issues

OvidiuFrantescu — Sat, 21 Jan 2017 18:14:28 GMT

Here is a copy/paste set of details regarding the raster file:

<Columns_and_Rows> 30001, 28766

<Number_of_Bands> 3

<Cell_Size__X._Y_> 0.013888889, 0.013888889

<Uncompressed_Size> 2.41 GB

<Format>TIFF

<Source_Type>Generic

<Pixel_Type>unsigned integer

<Pixel_Depth>8 Bit

<Colormap>absent

<Pyramids>level: 7, resampling: Nearest Neighbor

<Compression>LZW

<Mensuration_Capabilities>Basic

<Status>Permanent

<Extent><Top> 399.527777778 <Left> 0 <Right> 416.680555556 <Bottom> 0 <Spatial_Reference>WGS_1984_Web_Mercator_Auxiliary_Sphere

<Linear_Unit>Meter (1.000000)

<Angular_Unit>Degree (0.0174532925199433)

<False_Easting>0 <False_Northing> 0 <Central_Meridian> 0 <Standard_Parallel_1> 0 <Auxiliary_Sphere_Type> 0 <Datum>D_WGS_1984

<Statistics><Band_1><Build_Parameters>skipped columns:1, rows:1, ignored value(s): <Min>0 <Max>255 <Mean>253.5990525616515 <Std_dev.>14.24670552247798 <Classes>0

<Band_2><Build_Parameters>skipped columns:1, rows:1, ignored value(s): </Build_Parameters><Min>6<Max>255<Mean>253.6721133398082<Std_dev.>9.882874895876389<Classes>0

<Band_3><Build_Parameters>skipped columns:1, rows:1, ignored value(s): </Build_Parameters><Min>6<Max>255<Mean>250.7431879087078<Std_dev.>24.38898611239873<Classes>0

Thank you for any input you might have.

Re: Contour line issues

SteveLynch — Sat, 21 Jan 2017 19:01:24 GMT

This is a multi band image and only the first band will be contoured. It has values between 0 and 255

Re: Contour line issues

curtvprice — Sun, 22 Jan 2017 01:20:40 GMT

If it is a scanned map with contours on it the workflow you should use is to first georeference the image (preferably to match the coordinate system of the map) and then use ArcMap digitizing to capture the contours using 'heads-up' digitizing, that is, using ArcMap edit tools with your scanned map in the background, to digitize the contours with your mouse. This can be fairly time consuming but if the contours have any complexity this is probably the best way to capture the data in the long run.

Re: Contour line issues

OvidiuFrantescu — Sun, 22 Jan 2017 14:12:45 GMT

@Curtis Price - this is what I have done, but since this map covers an 18 mile long lake, and the contour lines are at 1 ft intervals it will take a long time to do by hand. Any advice on how to solve the problem automatically, or at least to reduce my work time?

Re: Contour line issues

DanPatterson_Retired — Sun, 22 Jan 2017 17:18:57 GMT

If the contours were derived from the raster... then they aren't representative of the data. The raster cell size is much larger than the contour spacing so you have spurious precision in your data.

If the raster coordinates are in decimal degrees...which still remains unanswered... It should be projected to an appropriate coordinate system. If you have to stick with feet for some reason make sure that the projected coordinate system has X,Y and Z in feet of if X and Y are in meters, make sure you account for the Z scaling if the elevations are in feet.

Once the data are projected using an appropriate cell size (ie. the foot equivalent of the cell sizes you report which are not in feet for sure), then you can use the contour tool. The contour spacing should not be the same size as your cell size, otherwise, you will get the same artifact that you are currently seeing.

Re: Contour line issues

NeilAyres — Mon, 23 Jan 2017 07:26:47 GMT

<Columns_and_Rows> 30001, 28766
<Number_of_Bands> 3
<Cell_Size__X._Y_> 0.013888889, 0.013888889
<Uncompressed_Size> 2.41 GB
<Format>TIFF
<Source_Type>Generic
<Pixel_Type>unsigned integer
<Pixel_Depth>8 Bit
<Colormap>absent
<Pyramids>level: 7, resampling: Nearest Neighbor
<Compression>LZW
<Mensuration_Capabilities>Basic
<Status>Permanent
<Extent><Top> 399.527777778 <Left> 0 <Right> 416.680555556 <Bottom> 0 <Spatial_Reference>WGS_1984_Web_Mercator_Auxiliary_Sphere

That, to me, looks like this tif hasen't been georeferenced at all, just defined as Web Merc

Re: Contour line issues

OvidiuFrantescu — Mon, 23 Jan 2017 14:44:43 GMT

So here is where I got now,

I started from scratch, captured over 250 print-screens to assemble my tiff raster, after ArcMap import I have georeferenced, projected, and reclassified the raster to display only two colors. Than using ArcScan I have extracted the center-line of each contour and placed them into a new line feature. All that is left now is to do some corrections of the resulted lines. Lots of tedious work for me, but nowhere near as much as digitizing the whole thing.

Thank you all for you input,

Ovidiu

Re: Contour line issues

curtvprice — Thu, 26 Jan 2017 23:34:41 GMT

If you can manipulate your contour raster into binary (1/0 or 1/nodata, where 1 are the contour line cells and 0 are the background) you can use automated methods in ArcScan. Worth looking into if you think it will save you time. These automated methods require that your raster is easily manipulated into this two-color format - Photoshop or GIMP expertise are helpful for this task.

What is ArcScan?—Help | ArcGIS Desktop

Re: Contour line issues

OvidiuFrantescu — Fri, 27 Jan 2017 14:24:54 GMT

Curtis,

I have ended up playing around with this extension, and it worked nice, especially the snap to raster function, but then it stopped working. After restart I could not make the ArcScan to recognize the raster so I can continue to digitize while snapping to the raster

I reclassified the raster to two values, 1/0, with 1 being the contour lines, and that is where I started.

Thank you,