Road Weather Information System
Environmental Sensor Station
Siting Guidelines
April 2005

5.0 Additional Considerations

5.1 Siting Considerations

The primary purpose for siting ESSs near roadways is to measure weather and pavement conditions on or adjacent to the roadway. These measurements are subsequently used to support management decisions and the prediction of future road and weather conditions detrimental to the safe operations of vehicles on the road. There are many obstacles that must be overcome for the proper siting of those stations.

The most difficult aspect of proper siting is to ensure that the sensor locations provide the specific road weather information required. This is not possible in all cases due to physical obstructions, safety, or security issues. In most cases, siting to satisfy the requirements for proper operation of one sensor can be accomplished, but the site may prove inappropriate for measurement of other desired conditions. In such cases, critical compromises or additional actions may be required to locate an ESS. Care must be taken to ensure compromises do not result in locating an ESS or ESS sensor in a location that will not be representative of the required road weather conditions. To avoid such situations, additional sensors or additional sites should be considered.

In addition to close proximity to the roadway, the sensors should be selected relative to the environment being monitored. If the intent is to measure winds of hurricane strength, then the instruments need to be of sufficient durability to handle the expected conditions. Additional actions may include adding sensors of identical types to an ESS in order to satisfy multiple requirements at a single location, e.g., installing a wind sensor on a bridge separate from one sited nearby in an open unobstructed area. The use of additional sensors will help ensure each is representative of the area or location of interest, but this will normally increase system costs and complexity.

Siting decisions should be based on the need for specific road weather information. Planning should address data requirements first; then address how to satisfy power and communications requirements.

5.2 Power

All sensors require some level of power to operate (e.g., signal voltages, sampling cycles, storage of data, and possibly heating elements). Power is also required for the collection of the data at the RPU and for transmission of the road weather data to its intended users. Power options include commercial power, wind power, or solar power with batteries.

The selection of the appropriate power option is dependent on the availability and dependability of the source. A commercial power connection is usually the most economical and reliable source of power. Solar power can support nominal loads but is typically not capable of sustaining heavy power consumption for heated sensors. Figure 9 illustrates two options for mounting solar panels. Care should be taken to ensure solar panels do not block or interfere with the operation of the atmospheric sensors. If the cost to physically connect to a direct, dedicated electric source is prohibitive, other options must be considered. The actual selection of the power source may subsequently dictate which sensors can be supported.

Figure 9. Options for Mounting Solar Panels

Cost becomes a factor in deciding whether to install less expensive sensors or communications requiring little power versus more capable sensors or communications requiring more power. Efforts should be made to ensure power considerations do not jeopardize the usefulness of the ESS. The use of solar power is more common in the western and southwestern United States where there is more ample sunlight. Commercial power, requiring less maintenance but considerably more installation costs, is used in most other areas. The use of wind power has been successfully implemented for a number of ESS installations in North Dakota.

5.3 Communications

Similarly, tradeoffs in the method and equipment to support timely and effective data communications (e.g., report frequency versus cost) may also have to be made, hopefully without compromising the usefulness of the ESS.

Communications options include hardwired telephone, cellular, copper wire, fiber optic cable, wireless, radio, microwave, or satellite. Important factors in the selection of the communication method and equipment include the amount of data that will be required to pass from the RPU. This will be a function of how much data are included in each observation (e.g., bandwidth considerations) and how often observations are transmitted (e.g., report frequency). For sites with low bandwidth requirements (i.e., no video camera and infrequent reporting), a polling system using telephone lines or some type of wireless communication may be more economical than hardwired options. In some cases, ESS siting decisions will be affected by line-of-sight and terrain shadowing considerations. For high data volumes, a hardwired communication system (wire or optical fiber) appears more appropriate, although installation costs could be increased considerably. DOTs should consider using “historical” polling of road weather data, if the RWIS RPU can support it. With historical polling, road weather data are stored in the RPU and are retrieved at set times (e.g., at the top of the hour and every 15 minutes thereafter). This process differs from polling the RPU to get only the current road weather observation. Employing historical polling enables the DOT to recover earlier road weather observations if communications’ failures interrupt the polling process.

A complete analysis of communication options and possible interfaces with the present or planned ITS should be performed early in the siting process. In some cases, ESSs can be located near other ITS devices (e.g., traffic counters, dynamic message signs, traffic signal controllers) to share power and communications costs. For critical sites, backup sources of power or communications may be needed. An analysis of communications should also consider the weather information requirements of partnering agencies, such as NWS. Partnering agencies may need weather observations at a greater reporting frequency than the DOT. Such requirements may influence the communications solution.

5.4 Aesthetics, Safety, and Security

In some cases, ESSs have been moved or painted in order to minimize how much they stand out from the surrounding terrain and vegetation. Unfortunately, following the siting criteria related to maintaining adequate distances from obstructions can result in a sensor tower that is very obvious. Pre-siting discussions with the surrounding stakeholders can possibly forestall any aesthetics problems.

Siting too close or too far from the roadway may seriously complicate maintenance procedures or unnecessarily jeopardize maintenance personnel safety. Installations too close to the pavement may make the data more representative of the actual roadway conditions but increase the hazard to maintenance personnel. Installations too far from the roadway may decrease the value of the data by making the data unrepresentative of the roadway environment. Extra security measures should be taken in areas where the threat of vandalism is present. These may include a security fence around the ESS, anti-climb panels, or even security cameras.

5.5 Periodic Siting Reevaluation

An important consideration of any ESS is the periodic reevaluation of the site. Over the years, construction projects, both on and off the roadway, and vegetation growth can change the representativeness and usefulness of ESS locations. A periodic reevaluation of the site is needed to ensure that the observation data from the site are still valid and that the metadata on the site are still correct. Obstructions should be reevaluated for interference and, if required, a new site should be determined or obstructions removed. Any changes resulting from this reevaluation should be included in updated entries to the site and sensor metadata in order to maintain a valid history of the site and its sensors. This reevaluation can be accomplished as part of an annual preventive maintenance program needed for sensor maintenance and calibration.

5.6 Siting Metadata

An important aspect of the effective use of road weather data from ESSs is the documentation and distribution of the site’s metadata. Metadata are basically defined as “data about data.” In the case of the RWIS ESS, metadata should include the station and sensor locations, the history of any changes in the metadata, and a representative index for the particular location and its sensors. The metadata are used to document the characteristics of each sensor and its siting to provide users a better understanding of what the sensor data really represent. The Federal Geographic Data Committee (FGDC) has developed standards for digital geospatial metadata.¹⁷ The referenced document provides “a common set of terminology and definitions for the documentation of digital geospatial data.” A companion document¹⁸ describes how and why a uniform set of criteria or standard was developed and presents it in more detail. David Hart and Hugh Phillips also have published a primer on metadata use.¹⁹ Currently, there are no corresponding uniform recommendations for RWIS ESS location and sensor metadata.

Various forms of metadata currently exist for ESSs that are not components of RWIS. The National Climatic Data Center (NCDC) maintains metadata for a number of its datasets. These datasets include: (1) Climate Reference Network dataset,²⁰ (2) NCDC Station History Database with approximately 30,000 stations and part of the NCDC Station History Information Processing System (SHIPS),²¹ and (3) NWS Cooperative dataset.²² The elements for the NCDC station metadata accessible through their Master Station History Report include station identification with a number of identifiers; location (e.g., country and state); latitude/longitude; station name; period of record; elevation; and station type. Elements for the cooperative sites include exposure, topography, and driving directions. The SHIPS database also contains more detailed information about the observations and instrumentation for some locations. In addition, development of a common set of metadata requirements for the NWS National Cooperative Mesonet, a part of the NWS Integrated Surface Observing System (ISOS) Program, is currently underway. The ISOS program may include RWIS as part of the national mesonet. To be included, RWIS data will need to have a minimum set of metadata for each site. Although still being finalized, this metadata set includes platform owner, station name and identifier, station coordinates, station elevation, observed elements with their units, station reporting frequency, and an observation timestamp description. All of these metadata items are included in the recommended ESS metadata set listed in Table 2. As metadata requirements become finalized for the NWS ISOS Program, RWIS metadata managers may want to adjust their metadata archive accordingly.

Currently, procedures for archiving ESS metadata vary from state to state. To maximize the application of ESS data within the transportation community and by other potential users, uniformity in metadata content and formats is encouraged. At a minimum, those items with a history of changes should be collected, centrally stored, and backed up whenever a change occurs. In addition to the collection and storage of the metadata, the metadata should also be readily available to any individual or agency that uses the data. The effective distribution of metadata is just as important as the distribution of the sensor data. This distribution of metadata is necessary to ensure that the user understands the characteristics of the road weather data, e.g., whether it is designed to monitor the conditions at a specific site for a specific local purpose or whether that data is designed for general surface transportation weather observing and forecasting.

Recommended metadata items for ESSs are listed in Table 2. Additional metadata items, listed in Table 3, can provide more useful information to road weather data users. Some items in the metadata sets in Tables 2 and 3 correspond directly to identification objects listed in the National Transportation Communications for ITS Protocol (NTCIP) Standard for ESS objects.²³

By establishing, maintaining, and sharing a robust metadata archive for ESS sites and their sensors, both transportation and other users will be able to effectively apply the ESS data to their particular weather and road weather information requirements.

¹⁷ Federal Geographic Data Committee, Content Standard for Digital Geospatial Metadata, FGDC-STD-001-1998.

¹⁸ Federal Geographic Data Committee, Content Standard for Digital Geospatial Metadata Workbook, Version 2.0, May 1, 2000.

¹⁹ Hart, D and H. Phillips, Metadata Primer—A "How To" Guide on Metadata Implementation, University of Wisconsin website.

²⁰ US Department of Commerce, National Climatic Data Center, United States Climate Reference Network (USCRN) Metadata Management - Survey to Operations (Draft). June 2003.

²¹ US Department of Commerce, National Climatic Data Center, Data Documentation for Data Set 9767B, Master Station History Report. December 31, 2002.

²² Viront-Lazar, A. and P. Seurer, Metadata for Climate Data, A Geographic Data Base Model for Station History, First IEEE Metadata Conference, April 16-18, 1996.

²³ US Department of Transportation. National Transportation Communications for ITS Protocol (NTCIP) - Object Definitions for Environmental Sensor Stations NTCIP 1204. November 23, 2001. Release of Version 2 is expected in 2005.

²⁴ Leroy, M. Meteorological Measurement Representativity, Nearby Obstacles Influence, 10th Symposium on Meteorological Observations and Instrumentation, American Meteorological Society, January 11-16, 1998.