As a key auxiliary device in the field of highway engineering surveying and monitoring, a road prism is an optical component used in conjunction with surveying instruments such as total stations and GNSS receivers. It accurately reflects the laser or electromagnetic signals emitted by surveying instruments, helping the instruments calculate the three - dimensional coordinates of the survey points. It plays an irreplaceable role in the entire life cycle of highway planning, construction, operation and maintenance. The following will provide a comprehensive analysis of road prisms from three dimensions: uses, application scenarios and advantages.

The core value of road prisms lies in providing a high - precision position reference for highway - related survey work. Specifically, their uses can be divided into three categories:

During the highway construction phase, prisms are key tools to turn "blueprints into reality". Surveyors will set up prisms at key points specified in the design drawings (such as road centerline points, edge points, slope toe points, bridge pile foundation center points, culvert inlet and outlet control points, etc.). The total station emits signals and receives the echo reflected by the prisms to quickly calculate the deviation between the actual points and the designed points. This guides construction workers to adjust the construction position, ensuring that key parameters such as road alignment, width, elevation and slope gradient fully meet the design requirements. For example, in subgrade filling construction, prism positioning can avoid problems such as subgrade deviation and insufficient width; in the pavement paving phase, prism measurement can accurately control the paving thickness, ensuring the flatness and strength of the pavement structure layer.

After a highway is put into use, it may suffer from subgrade settlement, pavement cracking, bridge displacement, tunnel lining deformation and other problems due to factors such as vehicle loads, changes in geological conditions (such as settlement and landslides) and natural disasters (such as heavy rains and earthquakes). At this time, road prisms will be fixed in key areas that need monitoring (such as soft soil subgrade sections, bridge supports, surrounding rock at tunnel inlets and outlets, etc.) to form long - term monitoring points. Surveyors regularly use high - precision surveying instruments (such as 0.5 - second total stations) to observe the coordinate changes of the prisms. By comparing the coordinate data of different periods, they analyze the deformation trend, deformation rate and cumulative deformation of the highway structure. Once the deformation value exceeds the safety threshold, an early warning can be issued in a timely manner to provide data support for highway maintenance and repair, and prevent safety accidents such as collapses and fractures.

During the final acceptance phase of highway engineering, prisms are used to complete the "completion surveying and mapping" work. Surveyors will arrange prisms along the road centerline and edge lines to conduct a comprehensive measurement of the actual alignment (plane alignment, vertical alignment), cross - section dimensions (subgrade width, pavement width, slope gradient), elevation and other parameters of the road, and generate a completion survey report. This report is not only an important basis for project acceptance (to determine whether the project meets the design standards) but also basic data for subsequent highway maintenance and reconstruction. In addition, before the construction of projects around the highway (such as pipeline laying and house construction), prisms are also used to measure the boundary position of the highway to avoid construction occupying the highway land.

II. Typical Application Scenarios of Road Prisms 

According to the different phases of highway engineering and work requirements, the application scenarios of road prisms can be divided into the following categories, each with clear application goals and operational characteristics:

After a highway project is approved, the first step is to conduct survey and design. At this time, prisms are mainly used for the layout of plane control networks and elevation control networks. Surveyors will select points with a wide field of vision and stable geology (such as mountain tops and the tops of fixed buildings along the road) along the project route, bury control piles with prism connection devices, and use total stations to conduct "triangulation" or "traverse survey" to determine the accurate coordinates of each control pile, forming a control network covering the entire project area. This control network is the basis for subsequent topographic surveying and mapping and route design. During topographic surveying and mapping, surveyors will carry portable prisms to conduct point - by - point measurements of the topographic features (such as mountains, rivers, farmland, and buildings) along the road in the field. They transmit the topographic data corresponding to the prism positions to the computer to generate a digital topographic map, which provides a basis for the selection of the route direction and the calculation of project quantities (such as earthwork volume).

The construction phase is the scenario where road prisms are most frequently used, covering almost all sub - projects:

• Subgrade Construction: Before subgrade excavation or filling, surveyors use prisms to position the excavation line, filling line and slope toe line of the subgrade to ensure that the scope and slope of the subgrade meet the design requirements. During the subgrade compaction process, prism measurement is used to control the thickness of the compaction layer (usually no more than 30 cm per layer).

• Pavement Construction: The pavement is divided into the subbase, base and surface layer. Prism setting - out is required before the construction of each layer. For example, during the paving of the asphalt surface layer, prisms are arranged on both sides of the road. The laser receiver on the paver receives the signals reflected by the prisms and automatically adjusts the paving height and speed to ensure the pavement flatness (usually requiring a flatness error of no more than 3 mm within every 3 meters).

• Bridge and Culvert Construction: In the construction of bridge foundations, prisms are used to position the center position of the pile foundation and the drilling depth. When installing the upper structure of the bridge (such as box girders and T - beams), prism measurement is used to measure the elevation and axis position of the beam body to ensure accurate docking of the beam body and avoid misalignment. In the construction of culverts, prisms are used to position the inlet and outlet positions and elevation of the culverts to ensure smooth drainage of the culverts.

• Traffic Facilities Construction: When installing guardrails, markings and traffic signs, prisms are used to position the centerline of the guardrails and the foundation position of the sign poles to ensure that the layout of traffic facilities complies with the specifications (such as the distance from the guardrail to the edge of the pavement and the height and position of the signs).

  
  

After the highway is opened to traffic, prisms are mainly used for long - term deformation monitoring and emergency detection:

• Long - term Monitoring Scenario: For highways in soft soil areas, highways with high - filled subgrades and bridges spanning rivers or valleys, long - term monitoring points will be set up (prisms are fixed on steel brackets or concrete piers, with windproof, rainproof and anti - collision functions). The measurement frequency depends on the risk level (usually once a month, and once a week in the rainy season or during periods of unstable geology) to continuously track data such as subgrade settlement and bridge deflection (vertical deformation of the bridge under load). For example, in soft soil subgrade sections, by monitoring the elevation changes of the prisms, it can be determined whether the subgrade tends to be stable (when the settlement rate is less than 0.5 mm/month for 3 consecutive months, the subgrade is considered stable).

• Emergency Detection Scenario: After the highway is hit by disasters such as heavy rains, earthquakes and landslides, it is necessary to quickly assess the damage to the highway structure. At this time, surveyors will carry portable prisms to conduct emergency measurements on the damaged sections (such as pavement collapses, slope slides and bridge support deviations). By comparing the prism coordinate data before and after the disaster, the degree of deformation is determined to provide a basis for formulating emergency repair plans (such as determining whether the road needs to be closed and whether the slope needs to be reinforced).

  
  

When the existing highway needs to be reconstructed and expanded (such as widening the pavement and upgrading the grade), prisms are used for two core tasks:

• Existing Road Data Collection: Surveyors will arrange prisms along the centerline and edge lines of the existing road to accurately measure the alignment, elevation and cross - section dimensions of the existing road, and generate a digital model of the existing road. This provides a reference for the reconstruction and expansion design (such as determining whether the existing road can be used and the position and scope of the widened part).

• New Route Positioning: In the construction of the new route in the reconstructed and expanded section, the use of prisms is similar to that in the construction of a new highway. They are used for the positioning and setting - out of the new subgrade, new pavement and new bridges and culverts to ensure the smooth connection between the new route and the existing road (such as the connection of the plane alignment and the connection of the vertical elevation) and avoid driving bumps or safety hazards.

  
  

Compared with other surveying auxiliary tools (such as reflective sheets and GPS antennas), road prisms have significant application advantages in the highway field, mainly reflected in the following four aspects:

Highway engineering has extremely high requirements for measurement accuracy (for example, the plane position error is usually required to be less than 5 cm, the elevation error is less than 3 cm; in bridge construction, the axis error of beam installation is required to be less than 2 mm). Road prisms, through special optical design (such as the use of total reflection prisms with a reflectivity of more than 95% and small signal loss), can accurately reflect the signals of surveying instruments. When used with high - precision total stations (with an accuracy of up to 0.1 mm + 1 ppm, that is, an error of no more than 1.1 mm per kilometer), the measurement error can be controlled at the millimeter level. For example, in the installation of bridge supports, prism measurement can control the horizontal displacement error of the supports within 1 mm, ensuring uniform stress on the upper structure of the bridge and extending the service life of the bridge. In addition, prisms have strong stability. In complex field environments (such as wind and vibration), the connection between the prism and the base is firm, and it is not easy to have position deviation, which further ensures the measurement accuracy.

Highway projects often involve complex terrains such as mountains, hills, rivers and deserts, and the construction and monitoring work needs to be carried out under different climatic conditions (high temperature, low temperature, heavy rain, sand and dust). Road prisms have excellent environmental adaptability:

• Strong Anti - interference Ability: Prisms reflect laser or electromagnetic signals, which are minimally affected by external light (such as strong light and backlight) and electromagnetic interference (such as high - voltage lines and radio signals). Compared with measurement tools that rely on visible light imaging (such as image measuring instruments), they can still work normally in strong light or at night. For example, during the construction of the asphalt pavement at noon in summer, direct sunlight will not affect the reflection effect of the prisms, and the measurement work can be carried out normally.

• Good Durability: The shell of road prisms is usually made of high - strength aluminum alloy or engineering plastics, and the surface is treated with anti - corrosion and anti - wear processes. It can withstand collisions, vibrations and harsh climates in the field (such as a temperature range of - 30℃ to 60℃, heavy rain erosion and sand and dust invasion). Some prisms also have a waterproof function (IP67 protection level) and can be used in rainy days or wading sections, with a service life of 5 to 10 years.

  
  

The terrain along the highway project is often complex, and the survey points may be located in areas that are difficult to reach, such as hillsides and river banks. The convenience advantage of road prisms is significant:

• Small Size and Light Weight: Common road prisms (such as single prisms and triple prism groups) usually weigh between 0.5 and 2 kg and are about the size of a fist. Surveyors can carry them around, making it convenient to arrange and adjust them in the field. Compared with traditional survey marks (such as large survey towers), the transportation and installation costs of prisms are extremely low.

• Simple Installation and Calibration: Prisms can be quickly connected to tripods and centering rods (a telescopic rod used to accurately center the ground points) through connecting screws, and the installation process only takes 1 to 2 minutes. The calibration work is also relatively simple. Only by setting the "prism constant" of the total station (the prism constant is an inherent parameter of the prism used to correct the measurement error) can the measurement accuracy be ensured, and no complex debugging by professional technicians is required.

  
  

From the perspective of long - term use, road prisms have obvious cost - effectiveness advantages:

• One - time Investment and Long - term Reuse: The price of a set of road prisms (including the prism body and connection devices) usually ranges from several hundred yuan to several thousand yuan, which is much lower than that of surveying instruments (such as total stations, which usually cost tens of thousands to hundreds of thousands of yuan). Moreover, prisms have strong durability and can be reused in multiple highway projects, reducing the measurement cost of a single project.

• Strong Compatibility to Adapt to a Variety of Equipment: The interface of road prisms (such as the M16 threaded interface) adopts industry - general standards and can be used in conjunction with mainstream brands of total stations (such as Leica, Topcon and South Surveying & Mapping) and GNSS receivers on the market. There is no need to purchase separate prisms for different equipment, which reduces the equipment procurement cost. In addition, some prisms also support the combined use of "multi - prism groups" (such as combining 3 or 6 prisms into a prism group), which can increase the signal reflection intensity and expand the measurement distance (the measurement distance of a single prism is usually 1 to 2 km, and that of a prism group can reach 3 to 5 km). It is suitable for the measurement work of long - distance highways (such as expressways and national highways).