topic Re: My flowlines all go uphill, why? in ArcGIS Spatial Analyst Questions

My flowlines all go uphill, why?

HillaryHudson — Thu, 27 Oct 2016 20:59:37 GMT

I have created a set of flowlines using the Fill, Flow Direction, Flow Accumulation, and Reclass tools (in that order) from the spatial analyst toolbox in 10.3.1. I then converted them to polylines from the original raster data. When I looked at my flow directions in the vector data, they were all going uphill. I would like some help trying to figure out what I did wrong. Thank you.

Re: My flowlines all go uphill, why?

ChrisDonohue__GISP — Thu, 27 Oct 2016 21:14:53 GMT

Some ideas:

Just to confirm - the linework is definitely going uphill, not due to the line direction being reversed when symbolized? Check the linework versus your rasters.

To convert from Raster to Vector, did you use Stream to Feature (Spatial Analyst)?

ArcGIS Help (10.2, 10.2.1, and 10.2.2) How Stream to Feature works

ArcGIS Help (10.2, 10.2.1, and 10.2.2) Stream to Feature

Also, if they are all uniformly reversed, as a workaround, one can reverse the line directionality.

ArcGIS Help (10.2, 10.2.1, and 10.2.2) Flip Line (Editing)

caveats - requires at least a standard license.

- alters the input dataset instead of making a new output, so back up your data before running.

Chris Donohue, GISP

Re: My flowlines all go uphill, why?

DanPatterson_Retired — Thu, 27 Oct 2016 21:15:07 GMT

got a picture? are they pointing upstream from the outlet?

Don't like it? then you can either Flip Line—Help | ArcGIS for Desktop if you have an appropriate license or just change the symbology in the layer so the arrow heads go in the other direction

Re: My flowlines all go uphill, why?

HillaryHudson — Thu, 27 Oct 2016 21:46:22 GMT

Hi Dan,

This is what a portion of the stream network looks like, and, yes, they are mostly pointing upstream from the outlet. There are an odd few that don’t. My stream orders all made sense and seem to follow the correct flow direction, the flow direction output itself, however, is backward. I used the from – to arrow direction so it is not a display issue. I am hoping that there is another way to correct them without having to flip one line at a time. I used a LiDAR dataset that was acquired by the county to generate the dataset. Thanks for your help!

Re: My flowlines all go uphill, why?

HillaryHudson — Thu, 27 Oct 2016 21:52:22 GMT

Hi Chris,

Thanks for your reply. Here is a screenshot of my flowlines showing how I symbolized the directional indicators. I have also gotten a second opinion and confirmation of the issue from a co-worker.

My stream orders all made sense and seem to follow the correct flow direction, the flow direction output itself, however, is backward.. I am hoping that there is another way to correct them without having to flip one line at a time. I used a LiDAR dataset that was acquired by the county to generate the dataset. Thanks for your help!

Re: My flowlines all go uphill, why?

HillaryHudson — Thu, 27 Oct 2016 21:52:59 GMT

Hi Dan,

Here is a more descriptive screenshot:

Re: My flowlines all go uphill, why?

ChrisDonohue__GISP — Thu, 27 Oct 2016 22:12:49 GMT

Not sure if this is a possible cause, but some of the results seem odd in ways other than the directionality you mentioned, at least compared to the Quad map displayed.

In the North, there are two areas where the derived stream lines seem to go parallel to the contour lines for a distance instead of traveling down the slope as expected.
Also, by the label "Upper Rock Tank" the lines go "horizontal" at first (which is a typical output result when derived streamlines cross a water body), but then seem to cross a ridge, which is not what one would expect.

Of course, the elevation data you used in deriving the stream lines may be different than the data used when the quads were constructed (time may have passed and the landscape may have changed, the resolution between the two datasets may be very different, etc), which may account for it. Still, it seems odd.

Chris Donohue, GISP

Re: My flowlines all go uphill, why?

DanPatterson_Retired — Thu, 27 Oct 2016 22:29:20 GMT

read the link I sent you carefully.... It does them all at once... And guess What!!! look at the example they use! Do you think that there might be an association???

arcpy.FlipLine_edit("C:/data/project.gdb/streams")‍

Re: My flowlines all go uphill, why?

DarrenWiens2 — Sat, 11 Dec 2021 12:20:09 GMT

Here's how you can flip lines with basic license:

>>> fc = 'lines'
... with arcpy.da.UpdateCursor(fc,'SHAPE@') as cursor:
...     for row in cursor:
...         new_part_arr = arcpy.Array()
...         for part in row[0]:
...             new_pnt_arr = arcpy.Array()
...             for pnt in part:
...                 new_pnt_arr.insert(0,pnt)
...             new_part_arr.insert(0,new_pnt_arr)
...         row[0] = arcpy.Polyline(new_part_arr)
...         cursor.updateRow(row)‍‍‍‍‍‍‍‍‍‍‍

Re: My flowlines all go uphill, why?

HillaryHudson — Thu, 03 Nov 2016 16:34:37 GMT

Thank you all for your helpful replies! I have been out of the office for most of the last week and I am finally getting a chance to sit down and review your responses. I will look these over and let you all know what I end up figuring out.

Cheers,

Hillary

Re: My flowlines all go uphill, why?

HillaryHudson — Thu, 03 Nov 2016 16:51:57 GMT

Dan Patterson, I just took note of the example of they used in the flip tool link you sent. It would appear that the flow direction tool may be wonky when you apply it to stream data, from what that example shows. From the feedback I am getting, it seems that the reason behind why the tool is not working properly isn't a known quantity, but flipping the lines is. Sigh. I wish I could get some answer regarding why the tool is having problems from the ESRI developers rather than just figuring out how to fix the bizarre output.

Re: My flowlines all go uphill, why?

HillaryHudson — Thu, 03 Nov 2016 16:54:34 GMT

Thanks Darren for the script on for the batch flip. It looks like that is going to be the solution that I use.

Re: My flowlines all go uphill, why?

Anonymous User — Thu, 03 Nov 2016 17:43:51 GMT

Hillary Hudson‌
Hi Hillary,

I can take a look at this and see if it is an issue.
Could you please zip the elevation data you are using and send that along with a brief description of your workflow to NRajasekar@esri.com ?

Thanks,
Neeraj

Re: My flowlines all go uphill, why?

HillaryHudson — Thu, 03 Nov 2016 20:00:08 GMT

Ok, I will do that. I am compressing the elevation data now, though I fear it will be too large to email. It is an entire watershed of LiDAR data.

Re: My flowlines all go uphill, why?

Anonymous User — Thu, 03 Nov 2016 22:46:36 GMT

Would you know if this data is publicly available?

In one of your comments you mentioned, "My stream orders all made sense and seem to follow the correct flow direction, the flow direction output itself, however, is backward." Could you please clip the flow direction output raster & input elevation raster, for a region where you can see that it is backwards? If you could send that, it might be sufficient enough for me to reproduce what you are seeing. In which case, you wouldn't need to zip the whole watershed elevation data and send that.

In the description of your workflow (Fill, Flow Direction, Flow Accumulation, and Reclass tools as you've indicated in your question) , please remember to mention what parameters options you use. If there are specific environments (other than defaults) you set for any tool, mentioning those would be useful as well.

If I can reproduce the issue with this data & information, we can look into it further .

Re: My flowlines all go uphill, why?

HillaryHudson — Fri, 04 Nov 2016 14:28:26 GMT

No, it’s not. It was acquired by Santa Fe County, NM and I do not believe they post their data to the web.

Re: My flowlines all go uphill, why?

HillaryHudson — Thu, 10 Nov 2016 15:09:01 GMT

Neeraj, I responded to your request for data last Friday in an email. My email server shows that it was sent. Have you been able to have a look at it? I communicated with you via regular email rather then through this thread since I do not see a way to attach file from this interface.

Re: My flowlines all go uphill, why?

HillaryHudson — Thu, 10 Nov 2016 15:34:12 GMT

I just got an email 5 minutes ago saying that my message was undeliverable due to the file size. I am not sure why it took almost a week to get that message....I will try splitting the data up and resending. I don't think I could make the subset any smaller and still have it be useful.

Re: My flowlines all go uphill, why?

HillaryHudson — Thu, 10 Nov 2016 16:54:06 GMT

I am resending my data as two emails.

Re: My flowlines all go uphill, why?

HillaryHudson — Thu, 10 Nov 2016 16:55:54 GMT

Unfortunately, even a very small subset of my LiDAR data is too small to send. Do you have an ftp site I can use? I do not have one I can use.