MDOT CORS Frequently Asked Questions

Michigan Spatial Reference Network (MSRN)
Michigan Department of Transportation
MDOT CORS
Frequently Asked Questions

The MSRN originally consisted of 12 stations located across the state collecting data continuously and has grown to over 85 stations. New stations are being added regularly.
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What is a Partner Site?

There are currently 29 partners consisting of counties and cities.(Wayne, St Clair, Macomb, Saginaw, Barry, Genesee, Washtenaw, City of Battle Creek, Dickenson, Muskegon, Charlevoix, Berrien, Ottawa, Livingston, Ferris State University, Gogebic Community College, Ogemaw County, Arenac County, St Joseph County, Department of Natural Resources, Alger Maximum Correctional Facility, Baraga Maximum Correctional Facility. IRESA School District, Van Buren County, Newaygo County, Grand Traverse County Surveyors and Grand Traverse County, Manistee County, Oceana County ). These entities have partnered with the MDOT in expanding the MSRN. Every station installed and included in the MSRN are built to the same specifications.
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What type of GNSS equipment do the MDOT CORS sites use and when will you support GLONASS?

The MSRN Station Consist of Leica GRX 1200 PRO receiver with Leica AT504 chokering antennas with radomes. MDOT has performed preliminary investigations with GLONASS and is constantly evaluating the situation. Some of the Stations are already GLONASS compatible receivers, however given the health of GLONASS constellation we will wait a while longer before fully implementing the GLONASS availability. Additionally, NGS does not support GLONASS in its RINEX format.
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What is the availability of the data and what type of data and sampling rates are provided?

The stations are designed to be in operation 24 hours a day 7 days a week. The data is collected at the receiver at a 1 second interval. The data is streamed in real time to a central location where the integrity of the data is monitored before it is written to a RINEX file. The MDOT website is designed to provide the user with a RINEX data set ranging from 1 hour to 24 hours containing sampling rates of 1, 5, 10, 15, 20 and 30 seconds.
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How long does it take to post the RINEX data from the MDOT CORS to the website?

The data for the previous hour is posted to the website 10 minutes after the hour. ( i.e. 7:00 a.m. to 8:00 a.m. data is available for download by 8:10 a.m.)
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How far back can I retrieve data for a particular site?

Data is available from our website for 2 years. Data over 1 year old is decimated to 30 seconds and concatenated into 24 hour blocks. We can not reproduce data which is more than 2 years old.
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How are these stations positioned?

The original 12 stations were positioned relative to other CORS stations (ITRF and NAD 83) and have also been positioned relative to the Michigan HARN (High Accuracy Reference Network) NAD 83-94 with cooperation of MDOT, Dept. of Natural Resources, Ferris State University, and private sector surveyors (Arborland Consultants, Ayres Lewis Norris & May Inc., Holland Engineering Inc., J.P. May and Associates, Orchard Hiltz & McCliment, Inc., Professional Engineering Associates, Spicer Group, & Williams & Works, Inc.). by occupying 59 "B ORDER or better" HARN points including the Coast Guard CORS sites and the 12 MDOT CORS sites. with the 2007 NGS adjustment, the CORS was held as control/

There is a discrepancy between the positions obtained from the CORS stations NAD 83 coordinates and the NAD 83-94 coordinates of the Michigan HARN. If you would like additional information regarding this discrepancy, there is a paper written by NGS which can by located at: http://www.ngs.noaa.gov/PUBS_LIB/HARN_CORS_COMP/eval_harn_to_cors.html

If using the MDOT CORS stations together with the Michigan HARN points, please be sure to use the coordinates for the CORS stations relative to NAD 83-94.
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What Antenna Height do I use when using an MDOT CORS site?

All the Elevations relative to the Michigan HARN (NAD83-94) are to the A.R.P. (Antenna Reference Plane a.k.a. Mechanical Reference Plane (M.R.P.)). If using the coordinates relative to the Michigan HARN, the antenna height would be "0.0 m" if the software takes into account the vertical offset of 0.11 m. This offset is from the ARP to the L1 phase center of the AT 504 choke ring antenna.

All Ellipsoidal heights published in relation to NAD 83-94 are listed to the A.R.P.

On the Sheets provided by NGS, they provide coordinates for the height to the L1 phase center and to the A.R.P..

Not using the correct ellipsoidal height or the correct antenna height will cause a significant error in your results.

For more information: http://www.ngs.noaa.gov/ANTCAL

Why are Michigan State Plane coordinates not available and published for the MDOT CORS sites?

When using CORS sites, stations from different zones are accessed which could lead to a potential for processing using the coordinates of different zones. All data processing software accepts Latitude and Longitude for its input.
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To obtain project control coordinates using the MDOT CORS sites, how long should my observation sessions be and how many sites should I use?

You should use at least the three closest CORS sites.

A good rule of thumb would be to observe for 2 hours per session minimum for distances less than 40 km. Add 1 hour for every 30 km thereafter. Tests have shown that there is not much change in accuracy regardless of baseline length for occupations over 4 hours.(distances are from the furthest CORS site you are using.) Individual results will vary depending on the equipment you are using. Observation Times are dependent on the density of MSRN stations in the area. The denser the spacing of the CORS,the less the observation times required.
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What is the difference between the broadcast and precise ephemeris and which should I use?

The broadcast ephemeris is the default ephemeris. This ephemeris is recorded by the GPS receiver in the field and is imported with the data. The broadcast ephemeris is only accurate from 2 to 5 meters. A precise ephemeris is computed after the fact and has accuracies ranging from 20 to 30 cm available after 24 hours, 5-10 cm available after 8 days, and 3-5 cm available after 14 days. The precise ephemeris files are available for download from: http://www.ngs.noaa.gov/CORS/Data.html. Use a precise ephemeris whenever you can.

How will the users be notified if the data will not be available?

If a system upgrade or maintenance will be performed causing a site or the entire system to be unavailable, a "news advisory" will be published on the website at least 24 hours prior. The word "MDOT CORS NEWS/ADVISORY" will appear on the bottom of the screen, simply click on the word to view the advisory. Please check the website before performing your day’s activities.
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Is there a cost involved with using the system?

No, the Michigan Department of Transportation uses this data for its daily activities and provides the data to users as a public service.
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Are the MSRN stations available for RTK operations and how do I obtain access to these RTK correction messages?

Yes, currently the MSRN is providing the RTCM (Real Time Correction Message) v2.3 correction message via NTRIP protocol (Networked Transport of RTCM via Internet). In order to access this message the user will need a wireless modem device that can access the wireless web. The modems come in all shapes and styles and through different wireless providers. MDOT does not recommend one provider over the other, the user must determine the coverage in his/her particular area to determine the best solution. The users also need to complete the RTK users agreement which is available as a link on the main page. Once the RTK users agreement is signed and returned to the MDOT, MDOT will provide the I. P. address of the system to the user.
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Why does MDOT only provide RTCM v2.3 and not RTCM 3.0, CMR+, or Leica data?

MDOT tries to be backward compatible as much as possible, 99 percent of RTK users can receive the RTCM v2.3 message. Recently more and more users have been requesting RTCM v3.0 and the other proprietary formats. In the near future, these formats may be available. We do provide RTCM 3.1 for network RTK solutions. See next FAQ.
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Do you provide Network RTK service?

MDOT provides 3 types of RTK broadcasts. The first one being Single Baseline (Classic) RTK, utilizing RTCM 2.3 via the NTRIP protocol. Network RTK is provided via NTRIP and RTCM 3.1 in the MAX (R2K2 Full) and i-MAX (R2K2 LITE) formats.The main difference between MAX and i-MAX is the processing is done at the ROVER for MAX and with i-MAX the processing is done at the servers.
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Are there any fees associated with the RTK access?

MDOT does not charge any access fees, MDOT provides all the data as a public service. The fees incurred with the RTK access are through the wireless providers.
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