Best Map Projection for modeling ENTIRE Ohio River

[Ray Davis]

Hi Folks,
The Ohio River Forecast Center is working on a large HEC RAS model that covers the entire Ohio River mainstem (about 1,000 miles long,) and about 100 miles both upstream and downstream of the Ohio/Mississippi confluence. That's about 1,200 miles of cross-section data! Yikes! Considering that we've already processed a large amount of data, I'm wondering what's the most appropriate map projection to use for processing DEM data.

Our multi-agency working committee agreed to use US Albers Equal Area (USGS version) several months ago. We decided against using UTM because our project area covers three zones (15, 16, and 17.)

Last week, we consulted with some folks from the USACE Hydrologic Engineering Center (HEC) and they feel we ought to be using a conformal projection like UTM. They thought we'd run into river-length distortions.

I did a quick distance check, comparing two lat-long locations projected to UTM zone 16 and to Albers. One point was at Pittsburgh, the other near Paducah. There was a ~3.2 mile difference.

UTM Zone 16 distance: 507.5496 miles
Albers distance: 504.3068 miles

I then checked a number of points on the Ohio River, connecting maybe 20 dots from upstream to downstream. I got a difference of ~1.4 miles.

UTM Zone 16 distance: 662.7969 miles
Albers distance: 661.4064 miles

We've got about 1,000 cross-sections between the upstream and downstream ends of this stretch of the Ohio. My gut feeling is that any length errors would be very small between the cross-sections. Further, the total length error along the mainstem of the Ohio River would be very small.

I'm certainly considering going with the HEC's suggestion to use UTM 16. However, I'd sure hate to have to reprocess 15-20 GB's of DEM data. Help!

Thanks,

Ray

[MattZ]

Raster Data can be reprojected in batch using ArcMap Raster Projection Tool. If you can dedicate a machine and disk space I think the manpower might be minimal.

[Ray Davis]

Hi MattZ,
I used that tool to project my data from Geographic (NAD83) to Albers US. That took a REALLY long time, but it did work. Is Albers an appropriate projection to use? UTM Zone 16 has been suggested, but I think there are map distortion issues with both projections. What projection(s) have you used, and how large are your study areas?

Thanks,

Ray

[MattZ]

I have no experience with project areas as large as what you are dealing with.

The smartest guy I know dealing with coordinate systems and projections is Norm Olsen, the founder of Mentor Software, Inc. He wrote a coordinate conversion program called Tralaine. He’s a real nice guy and I think he would at least respond if contacted.



Norm Olsen

888-ASK-NORM

Norm.Olsen@MentorSoftwareInc.com

[krista1223]

Hi Ray,
Our company is doing a model for one of the tributaries to the Ohio River, I was wondering if you guys had made hydrographs for the Ohio at different locations downstream. We would be interested in seeing how late the peak occurs to determine our downstream watersurface elevations.

If you have anything and would be willing to share, please let me know.

Thanks,
Krista Greer
RK&K Engineers
kgreer@rkk.com

[Ray Davis]

Hi Krista,
I think that Tom Adams from my office spoke with you about this topic. As he explained, we don't have any results to share at this time.

Thanks,

Ray

[Ray Davis]

Hi all,
NAD83 Albers turned out to be the best projection to use for our project. Length errors were minimal when compared to other projections. Our project area has primarily an East-West extent. UTM would likely be a more appropriate projection for project areas that are predominantly oriented North to South.

Thanks for your help everyone!

Ray Davis

Hydrologist
NOAA National Weather Service
Ohio River Forecast Center
Wilmington, OH