The procedure & advantages of auto level surveys
In this civil engineering article, you will get detail information about Auto level surveys. This type of survey is normally applied to finish cross-sectional and longitudinal surveys. At least two field personnel are required to conduct this survey. There should be a base map to point out positions of cross-sectional and longitudinal surveys. If there is no site elevation datum, then it is essential to set a ‘project datum’ (generally these datums contain a base value of +100) and evidently document on the base map.
To conduct the auto level survey, the following minimum equipments are required :
- Base map
- Auto level
- Tripod (to mount the auto level)
- Rod (to calculate ‘elevations’)
- Tape measure (long tape measures, 100 or 300 feet, work best)
- Clipboard and pencils
- Digital camera (to recognize the features inside your cross-sections)
Prior to start the field work, it is necessary to examine all the equipments so that they function properly.
Repairing and/or changing faulty field equipment beforehand, will save huge time.
Prior to commence the field work, theoretically lay out required numbers of cross-sections (and the spans of the cross-sections), the spans of your longitudinal survey, and any other physical measurements.
This preliminary plan will facilitate to gather adequate data as well the amount of time in the job site. With this site visit, you will get details about the site, recognize any access issues (like locked gates, private property, etc.). It will also facilitate you to evaluate the amount of vegetation onsite which may create problems at the time of your survey work.
After that, the auto level survey will be started with the following steps:
Also Read : Various types of scales in surveying
- Start at the project benchmark/datum. As the primary measurement recorded by the auto level is elevation, ensure that your initial measurement records this datum. It is treated as the basis for all your future measurements.
- Set your auto level in an area where the project benchmark and your first cross-section location can be evidently viewed. It will facilitate to set up the elevation at the start of your cross section profile. If there is not a clear line of sight from the benchmark to your cross-section location, it is necessary to traverse (settle a temporary turning point that will be surveyed in from the project benchmark with your first auto level setup) from the project benchmark to the cross section location with a turning point.
- As soon as the auto level instrument setup location is set, arrange the tripod (ensure to tightly sink the tripod legs into the ground to fix the tripod at the time of your survey). Attach the auto level and level the tripod and auto level by altering the tripod legs to obtain the level bubble adjacent to the ‘level circle.’ Fine leveling corrections are done with the leveling screws on the auto level.
- After the instrument is settled, establish the elevation at the horizontal line inside the eyepiece. It will be the foundation from which the future measurements will be started. To set up your instrument elevation, settle your rod on the project datum (ensure the rod expands high enough to become noticeable to the auto level). Read the vertical distance from the rod, this distance when included with the project datum, it provides you the elevation at the instrument center. All future measurements should be deducted from this number to produce the surveyed ground elevation.
- As soon as the elevation at the starting point of your cross section is established, it is required to expand your measuring tape across the alignment of your cross-section. The horizontal distance can be applied (calculated from your tape measure) to work out the cross-section station (starting with station 0+00).
- Prior to conduct the cross-section survey, begin your cross-section field notes sheet. It is essential to recognize the field personnel, the data, the equipment, and it will be the location to record the station and elevation readings (which will be then applied to graph and plot your cross-sections). The field notes sheet should contain the following columns: Station, Rod Reading, Ground Elevation, Notes. The purpose of the ‘notes’ column is to recognize any special comments on the surveyed point, like ‘boulder,’ ‘log,’ etc.
- As soon as your tape measure is expanded (with minimal sag) across your cross-section alignment and your notes sheet is settled, conduct the survey. At each preferred station, settled the rod on the ground and rock it slightly backward and forward. As most field rods do not contain level bubbles installed on them to make sure that the level of the rod is accurate, the slight rocking back and forth of the rod will facilitate the person at the auto level to precisely read the rod by recording the minimum number detected (that happens when the rod is ‘level’). Record the Station number (that is equal to the distance calculated on your tape measure, with the “0” place signifying hundreds and the “00” signifying tens, so for instance, 150 m would read as Station 1+50 and 54 m would read as Station 0+54).
- While choosing surveying locations, make sure that adequate number of points are gathered to define the topography of the cross section properly. While accomplishing the survey work, the straight lines should be drawn among the surveyed points. Normally, composite sites need extensive survey work as compared to simple sites with little topographic intricacy. When your cross-sections are required to be integrated with detailed numerical models, it is necessary to gather more information.
- It may also require to record the water level in the cross-section to be surveyed. If the water level is recorded, don't forget to note the date and time, as water levels fluctuate with time.
- At the end of your survey, it may require to set up monuments for doing re-survey in the future. Common cross-section monuments are rebar lengths (~1 m) headed with a plastic rebar cap. It is also required to photo-document your cross-section to facilitate drafting your cross-section.
- The final step in the survey process involves ‘closing out’ your survey by re-calculating your starting elevation mark. It indicates that there are no errors in your survey due to movement of the auto level.