Field Manual 3-34.331 TOPOGRAPHIC SURVEYING 16 January 2001

TOC Chap1 2 3 4 5 6 7 8 9 10 11 AppA AppB AppC AppD Gl Bib

Chapter 3

SURVEY RECON

The recon party must consider special factors, as determined by the objective of the survey, and the methods, techniques, and equipment that will be employed. This chapter discusses general recon considerations. Survey methods and techniques are discussed in the following chapters.

RECON REQUIREMENTS

3-1. A proper survey recon includes 

• Gathering all existing survey data about the target area.
• Testing and determining the usability and visibility of existing stations.
• Selecting sites for the main and supplemental stations.
• Determining the monumentation requirements.
• Collecting terrain and climatic information.
• Arranging for access to private or government property.
• Checking on the availability of lodging, mess, medical, maintenance, and other required support.

3-2. Interreceiver visibility is not required for GPS surveying. Stations can be set according to network-design principles rather than traversing around buildings or mountains. The only requirement for receiving GPS signals is a clear view of the sky. Sources of electro-magnetic interference and tall buildings should be avoided. Choose a station with no obstructions above an inclination of 15� to 20�. Draw a station obstruction diagram to assist in the planning of GPS sessions. Verify the station's accessibility and then draw maps with directions to the stations and mark each station clearly. The field crew will be in a hurry to set up when they arrive, and unmarked stations can waste valuable time.

3-3. During special surveys when the need to locate the position of a point that cannot be occupied arises, triangulation is necessary. This technique places special demands on the recon party. The mathematical computations place stringent requirements on the size and shape of the geometric figures that are used to determine coordinates. For this reason, the location of the stations will normally be dictated to the field-recon party, based on the results of the office recon. The recon party must ensure that the observation stations which form the baseline are intervisible. A thorough knowledge of triangulation criteria is absolutely necessary.

3-4. The demands for a traverse recon are less stringent than for triangulation. Ensure that both the rear and the forward stations are visible from each proposed station. Wherever possible, distances between stations should be uniform. In control surveys that may become part of the US National Control Network, the SSGCN must be satisfied. Spacing between stations will be dependent on the EDME available.

3-5. An EDME traverse recon requires intervisibility between stations. The minimum and maximum allowable distances are based on the EDME characteristics and the clearance above possible obstructions. Use of infrared EDME will be dependent on the weather.

3-6. Differential leveling should follow routes containing the least amount of change in elevation between BMs and individual setups. The routes will frequently follow roads with moderate traffic, so care must be taken to ensure the safety of the leveling party.

3-7. A trigonometric-leveling recon is accomplished when a traverse recon is performed. When given a choice between a relatively level, a greatly elevated, or a depressed observation, select the relatively level observation. Failure to accurately level the instrument will cause a greater error in an elevated or depressed observation.

3-8. Recon for other control methods will vary according to the physical characteristics and limitations of the equipment or system used. No matter what system or equipment is being used, the proposed station must be accessible and the proposed station must be able to be included in the local survey-control scheme. Stations occupied by PADS must not exceed the maximum distance and time from the initializing station.

RECON-PARTY COMPOSITION

3-9. The recon party will vary in disposition and number according to the method of survey, the type of terrain, the available transportation, the extent of the survey, and the density of control required. The chief of the recon party is normally the section leader. The recon party usually consists of two to five personnel. As a minimum, it will include the survey-party chief and the section leader. It is also helpful to include personnel who will be instrument operators. The most qualified unit members should be assigned to the recon party, because a properly designed recon will result in a survey project that is accurate, complete, and expeditious. The recon party should be thoroughly briefed on the project instructions and the specifications of the survey mission. Recon is accomplished in three phases office recon, field recon, and recon reports.

OFFICE RECON

3-10. The office-recon phase includes the gathering of existing data and a study of applicable maps. This phase will be completed before the start of the field-recon phase.

3-11. During the office-recon phase, the first step is to gather all existing data on the area to be surveyed. Depending on the area, there may be a number of sources that maintain some type of reliable survey data. The existing data will usually consist of trig lists, station description cards, and aerial photographs or maps. Trig lists come in many forms, depending on the publishing agency. A trig list may be compiled on DA Form 1959, horizontal-control data booklets from the National Geodetic Survey (NGS), or a computer printout of coordinates. Sources of information include 

• Local Army units (such as map depots, FA target-acquisition (TA) units, SIC, and survey units).
• The NGS and the US Geologic Survey (USGS).
• USACE district offices.
• The US Department of the Interior, Bureau of Land Management.
• State and local government civil-engineering or survey offices.
• Other nations. Existing data is sometimes received from the national agency charged with the mapping of that nation. Local municipalities and city governments also have survey information in their engineering or land-planning offices.
• Continuously operating reference stations (CORSs) for CONUS.

3-12. Regardless of the information source, all trig lists (officially classified or not) must be safeguarded. Once secured, this information should be maintained as a database for that area since it may be necessary to conduct additional surveys in the same or an adjacent area.

3-13. Do not evaluate the existing material until all material has been assembled and the information has been annotated on the available maps or aerial photographs. Plot the required SCPs from the project directive, and then evaluate the usability of existing controls. Compare the required control method with the existing control method to determine if additional, basic control is needed. It is possible that many required stations may be eliminated because adequate control already exists. For those required stations that must be established, a tentative route of survey is annotated on the maps.

FIELD RECON

3-14. The field-recon phase is different for each survey project. A party chief must consider and apply the lessons learned from previous projects. The methods and techniques can be changed to suit the conditions of the current project. A successful party chief will also employ the knowledge and ingenuity of the survey-party personnel.

3-15. When time permits, the party chief and one other person will conduct a preliminary field inspection of the area. When gathering information concerning the area to be surveyed, include terrain types, tree heights, road width, road surfaces, spacing between roads, microclimate (fog, haze, and heat waves), and any other factors that will affect distance measuring and intervisibility between proposed stations. The inspection may be conducted using vehicles, helicopters, or airplanes. The results of the inspection will determine the scheme and route for the survey.

3-16. In areas where control is to be extended or established, there may be control stations from earlier surveys that must be recovered and verified. These stations should have been identified and annotated on overlays during the office-recon phase and will serve as starting points for proposed GPS networks, traverse lines, or level lines. The existing stations should be located, described, and verified for accuracy, before using them for extending control.

3-17. Existing control stations (and their establishing surveys) follow similar patterns. Recognizing and associating the patterns with the terrain types will assist the surveyor in locating existing stations.

• Triangulation stations are usually found on the highest point of a hill or a mountain. In areas of little relief, the stations may be located at prominent points or sites where a tower could have been easily erected.
• BMs and traverse stations are typically located along roads, railroads, pipelines, or other transportation routes, which permit intervisibility and accessibility. BMs and traverse stations may also be found along waterways, rivers, canals, and coastlines.

3-18. In some areas, urbanization has changed road or drainage patterns. In rural areas, land may have been cleared and cultivated or fields may have become overgrown or reforested. Gather and consider all available information when searching for a station.

3-19. Trig lists, control cards, and control bulletins contain brief descriptions and sketches of stations. The information may be outdated or insufficient for a final product but will permit surveyors to locate the general vicinity of the station. The final steps in locating the station will involve the use of distances and azimuths from the reference marks (RMs) to the station.

3-20. Previous survey data may include survey schemes, overlays, or plots depicting the relative position of the stations in the general area. After one or more stations have been recovered, the other stations may be roughly plotted and located using a magnetic compass and either intersection or resection methods.

3-21. Aerial photographs may be used if the station to be recovered can be identified on the photographs. Using features that are permanent and prominent on both the photograph and the ground will permit surveyors to reach the station site.

3-22. Maps with the plotted coordinates of the station will permit surveyors to identify the route of travel to the station. Maps will also assist surveyors in determining the station's accessibility.

3-23. Local information sources include local surveyors, public-service officials, construction companies, and landowners. Local sources may be the only means of locating a station if the area has dramatically changed since the other sources of information were published.

3-24. Verification of a station must be performed before using the station. Where only one other station is intervisible, a check-distance measurement can be performed using the GPS or a conventional method. Where two or more stations are intervisible, check-angle observations or GPS measurements can be performed. After the measurements and observations have been performed and reduced, they will be compared to the published information. If the results agree within the overall specifications for the survey project, the stations may be used.

3-25. New station sites will be selected after all existing stations have been recovered, described, and verified. The new stations will be placed where required to complete the scheme of the survey.

3-26. Correct selection of a new station site will save time and expense and will prolong the life of the new station. Consider the following paragraphs when selecting a new station site.

3-27. Permanency. Monuments (also referred to as marks or markers) can be permanent or temporary.

• Permanent monuments. Permanent monuments are set in a relatively stable material or structure for the purpose of preserving the location of either horizontal or vertical control. Consider another site if the proposed site may experience disturbance or land development. Since there are a wide variety of possible situations that may be encountered when setting a monument, it is impossible to address them all. The ultimate selection of the site is at the discretion of the monument setter.
• Temporary markers. Temporary markers are the same as permanent monuments except that the preservation time required is less. Temporary markers shall consist of a 1- by 2-inch wooden hub (or larger) with adjacent guard stakes, a copper nail and washer, or a temporary spike that is set in relatively stable material.

3-28. Security. Foremost on the list of considerations is a monument's susceptibility to damage or destruction. It is necessary to anticipate any construction that might occur in the area. Frequently, marks that are set in asphalt surfaces are paved over periodically. Marks that are set off the edge of the asphalt surface will stand a better chance of survival.

3-29. Accessibility. Accessibility of the marks should be evaluated in selecting the site. If the mark cannot be found or conveniently occupied, its worth is questionable. Determine if there are nearby objects that can be used as references. Distances and directions from prominent reference objects are used to locate a mark. These distances and directions are referred to as lines of position (LOPs). The prominent objects are referred to as origins. At least two LOPs are required to describe a point. The closer to perpendicular that the angle at which the LOPs intersect, the more accurate a position can be described.

3-30. Stability. All marks are subject to the effects of geologic and soil activity. Vertical-control marks or BMs are particularly vulnerable because this activity results in vertical movements much more than horizontal motion. Selecting advantageous topographic features (such as the crests of hills) will increase soil stability and decrease frost heave and the consistency of the soil will tend to be more firm. Also consider the soil-grain size, and when possible, choose a site with coarse-grained soils. Fined-grained soils (such as clays) are susceptible to high moisture content, which can be affected by frost and erosion.

3-31. Safety. If a mark extends below the ground, there is a chance of encountering underground cables or pipes during installation. Evidence of underground utility lines can often be observed at the surface. Waterlines are marked by valve boxes, and in structures newer than 1960, the utilities are likely to be buried. Avoid digging near light poles, phone lines, or electric and gas junction boxes.

3-32. Visibility. Select sites that provide maximum visibility above the horizon, plus 15�. Any obstruction above 15� will potentially block satellite signals. The ideal site should have visibility in all directions above 15� however, in some locations at specific times, an obstruction in one or two directions may not affect the ability to use the site for GPS surveying. Existing BMs should be used as GPS monuments as often as possible. New marks should be located as close as possible to a known vertical control. Maximum effort should be made to locate all GPS-type monuments within 100 feet of easy access to ground transportation.

3-33. Names will normally be assigned by the customer (for example, the project name or number followed by the sequence number of that station in the scheme-of-control extension). Names should be an alphanumeric symbol that is stamped on the respective disk marker. The name that appears on the control point for publication purposes should be the same as the name that actually appears on the mark. Old stations that are reestablished will be given the previous name with a numerical suffix added (such as Boulder number 2). In the absence of guidance from the customer 

• Use the name of a nearby geographical feature.
• Use short names (maximum of 25 characters, including spaces).
• Include the name of the agency or unit that set the mark if it is not precast.
• Make sure the station name is spelled correctly on all documents.
• Do not use special characters such as periods, commas, slashes, or equal signs.
• Do not include nondescriptive terms such as spike or nail or personal names.

3-34. Permission must be obtained before conducting a survey on any private land. The survey-section sergeant or the party chief, working through the local Judge Advocate General (JAG), will contact and negotiate with landowners for access to prospective station sites. Written permission to enter the land is preferred because it is documented. The local JAG will assist in this matter and will help keep the military out of potential trouble.

3-35. US. The recon and survey parties should have a right-of-entry letter to the overall area from their HQ. This letter does not entitle the survey team to access private property or restricted areas without further permission. When the landowner is contacted, a full explanation of the work to be done is given without any attempt to conceal any inconveniences or damage that may arise. Government regulations concerning damage claims should be explained when necessary. In the case of an absentee owner, who cannot be reached in person, a letter explaining the work and asking consent to access the property should be mailed.

3-36. Other Nations. When working in other nations, the appropriate officer of the US embassy within that country will generally negotiate the right-of-entry letters for overall areas within that country. However, a right-of-entry letter or approval from the host nation is not always sufficient for access to all public lands within the national boundaries. It is sometimes necessary to contact the local officials where the work is to be performed. Agreements will be conducted according to local customs. Some countries consider an oral agreement, or any statement that could be construed to be an oral agreement, to be contractual and binding. Any transfer of assets (material or otherwise) require close coordination with the JAG.

3-37. The setting of stations should be accomplished during the recon phase. The selection of the monument type is based on local site conditions. The types of marks to be used for vertical and horizontal control are a function of the order and accuracy of the survey, the intended use of the data collected, and the site conditions.

3-38. A variety of standard monuments (described below) are currently available for use as surface station marks. On projects conducted for NIMA or the USACE, standard NIMA or USACE disks should be used. The disks are set in the top of a concrete post or another appropriate monument. Each survey method has individually designated disks. These station marks must be as permanent as possible, intelligently placed for present and future use, and safe from damage. In cultivated fields or in pastures (which may later be cultivated), the owner's permission should be obtained to build rock cairns or to set guard or witness posts around monuments.

3-39. Subsurface station marks are used for first-, second-, and third-order stations. Pipe, rebar, and sectional rods are considered subsurface marks and aid in the relocation of disturbed marks. Where bedrock is exposed and a Type C monument is used, no such mark is feasible, and the drill hole itself is sufficient.

3-40. The type of monument used depends on the terrain, the climate, and the soil composition. Engineer manual (EM) 1110-1-1002 identifies specifications for survey markers and monuments. Monuments can be subdivided into two general categories standard and nonstandard.

3-41. Standard Monuments. Standard monuments use some form of standard survey disk. These disks may be brass, bronze, aluminum, or other alloys. Tables 3-1 and 3-2 suggest the type of monument to be used according to required vertical and horizontal accuracy (USACE standards). A Type G monument is sufficient for all third-order surveys, both vertical and horizontal.

• Type G monument. This classic, standard monument is made completely of poured concrete with a disk set in the top of the concrete (Figure 3-1). These procedures and dimensions are for a second- or higher-order monument. A Type G monument is constructed by excavating a hole that is 15 centimeters in diameter and 60 centimeters deep. In areas where the maximum frost depth is greater than 60 centimeters, the hole should be 30 centimeters below the frost depth. The disk should be driven onto a pipe, a rod, or a number 5 rebar that is 120 centimeters long. The pipe, rod, or rebar assembly is then driven into the center of the hole until the top is slightly above the surface. The hole is then filled with concrete, which must not cover the disk. The use of pipe, rod, or rebar is optional. The disk may be pushed directly into the fresh concrete, but a magnet must be placed in the concrete if the bar is omitted.

Figure 3-1. Type G Monument

• Type C monument. Sound bedrock is the most desirable location for a BM, as illustrated by Figure 3-2. It provides the most stable setting in terms of underground activity and potential disturbances. Always use bedrock when a suitable outcrop exists. Use a star drill to make a hole about 2.5 centimeters wide and 6 centimeters deep to receive the shank of the marker. Fill the hole with epoxy resin and insert the disk, with the resin slightly built up around the edge. When a solid bench or ledge is covered with a few feet of top soil, the subsurface mark should be in the ledge and a concrete monument should be set above it to protrude above the surface.

Figure 3-2. Type C Monument

• Type A monument. Use a Type A rod monument (Figure 3-3) when sound bedrock or substantially stable structures are not available. The monument provides the extra horizontal stability required for 3D surveys, which makes the monument a suitable GPS mark. Refer to EM 1110-1-1002 for details on installing a Type A monument.

Figure 3-3. Type A Monument

NOTE: Type A monuments are used in marshes. Type G monuments are used in permafrost areas.

• Precast monument. To eliminate the need for mixing and pouring monuments at the site, precast monuments may be used if the project specifications permit. These precast monuments are fabricated at the base station or camp and are constructed with the equivalent dimensions listed for poured concrete. If a subsurface mark is required, it is placed as identified above with a carefully plumbed precast monument.
• Commercial monuments. A number of commercial monuments are available that can be considered standard monuments. These are generally metal or plastic rods (with a disk affixed to the top) that are driven into the ground.
• RMs. RMs are usually set in the same type of monument as the main station, but they can be made smaller. The number of RMs used depends on the survey method. In triangulation and traverse methods, at least two, but normally three, RMs will be set for each station. These marks should be located within 30 meters of the station and at intervals of about 120� around the station. No subsurface marks are used with these marks. RMs should be located where they are least likely to be disturbed and where direct measurements can be made to them from the station. It is permissible to use drill holes or chiseled marks in rock outcrops.
• Azimuth marks. Azimuth marks are established in connection with SCPs to furnish an azimuth that will be available to local surveyors from an ordinary ground-level instrument setup. These marks are used in the extension of control from the station. The readings to azimuth marks are observed as part of the traverse method. Azimuth marks are permanent monuments that are placed in a prominent and safe location and more than 400 meters but less than 3 kilometers from the triangulation station. Prominent, permanent man-made structures may also be used as azimuth marks (for example, the light on the top of a water or radio-station tower or the cross on a church in a nearby town).

3-42. Nonstandard Monuments. These monuments can take many forms and, if properly installed, provide for a good, permanent control station. Some examples are 

• Expended shell casings (7.62 to 105 millimeters) embedded into a concrete post as prepared for standard monuments.
• Sections of rebar or pipe driven into the ground with a concrete collar poured around the upper 0.3 meter.

• Witness post. A witness post is a sign or stake driven into the ground next to the station or RM.
• Guard post. A guard post is emplaced around a station that is susceptible to damage from ground traffic. They are generally large wood stock (8 inches by 12 inches by 8 feet) or expended steel (such as sections of railroad rails or heavy pipe). They are usually set 1 to 1.5 meters into the ground and secured with concrete.

3-43. The recon party will prepare a description and sketch of all newly established permanent and temporary stations and all stations recovered. Stations recovered, but not used, must also have a description completed. The description and sketch will be done on DA Form 1959 (Figure 3-4) or in an appropriate field book. The field record is done in free hand using vertical gothic lettering. A final DA Form 1959 should be typed and kept with the official records.

Figure 3-4. Sample of DA Form 1959

3-44. Provide a narrative report (compiled at the station site) containing all the information necessary to expeditiously locate the station. The description should enable someone totally unfamiliar with the area to go, with certainty, to the immediate vicinity of the station. In conjunction with a sketch, a positive identification of the station and RMs should be possible. Avoid repetition where possible. The description should be brief (to the point), logical, and include the following information.

3-45. The authorized recovery notes are as follows:

• New station. This is a newly established station for which no description exists.
• Recovered as described. This is a station that is recovered exactly as described. All marks are in good condition, the distances and directions are verified, and the sketch and description are adequate. The statement alone is sufficient for the recon recovery card. Transcribe the old sketch and description onto the new control card.
• Recovered. This is a recovered station with changes that make the old sketch and description inaccurate or inadequate. Complete a new card and make a new sketch and/or description of the station. Report any alterations to the station or RMs and describe the altered marks and new measurements of the referenced distances and directions. An effort should be made to improve all sketches and descriptions.
• Not recovered. This is a station for which no positive evidence of existence can be found after a diligent search has been made.
• Destroyed. This is a station at which there is positive evidence that the station did exist, but the station and its RMs have been so mutilated that it cannot be replaced within 1 centimeter of its original position. The individual making the recovery and writing the description must use judgment in determining the status of a station. A station may be destroyed for precise purposes but still be valuable for surveys requiring less accurate control (for example, gravimetric, magnetic, or astronomic surveys).
• Reset. This is a station at which the monument and/or station marks have been replaced so that the mark is within 1 centimeter of its original position. A station is reset only from subsurface and/or RMs that have not been moved from their original positions. The task of resetting monuments may be assigned to the recon party.
• Disturbed. This notation is generally used only with reference to vertical control points. It is a station at which the monument is physically present, but it has been so moved that it has lost its value as a vertical control point within the accuracy to which it was originally established.

3-46. This information follows the recovery note. It identifies the location of the station area on a map in relation to cities and towns, bridges, and other major landmarks. The political subdivision should also be stated.

3-47. This describes the route to the station site. The description should start from an easily located point such as a public building, a park, a main-road intersection, or any other permanent landmark that is identifiable both on the map and on the ground. Distances between checkpoints on the route are given in miles and tenths of miles or kilometers and meters. Changes in route direction are given as both left or right and east (E), west (W), north (N), or south (S).

3-48. Describe the exact location of the mark in relation to readily identifiable RMs. List the magnetic azimuth and the distance from the reference point to the station mark.

3-49. Describe the actual mark (for example, drill hole, bronze disk, or chiseled mark in stone) and the exact stamping on the mark (agency, year, and type of station). Note if the station mark is above or below the ground's surface.

3-50. Describe RMs in the same manner as the station mark. Include the distances and directions measured from the station mark.

3-51. Describe the azimuth mark in the same manner as RMs. The distance is usually approximated rather than measured.

3-52. Describe the field of view from tripod height. For example, the view is unobstructed in all directions except south and the trees (60 feet high, 300 feet from the station) obstruct the view between the magnetic azimuths of 170� and 215�.

3-53. List any important information about the station site (which is not covered elsewhere) in the notes at the bottom of the description. This may include a photo number and mission (if applicable), danger areas, or access concerns.

3-54. The sketch should be clear and simple and contain only enough detail for positive identification of the station. In general, it should contain the 

• Features of a permanent nature. Show the features around the station with enough detail so that they will not be confused with other similar features. For example, many road intersections and hilltops look alike. Extend the sketch slightly so that the characteristic features become evident. When there is little detail available, make a rough contour sketch. Use only standard topographic and military symbols on the sketches.
• Scope and scale. Judgments on what features are actually required to identify the station and the individual's ability to draw will usually govern the scope of a sketch. Normally, a sketch should include the area within a radius of 200 feet to 1/2 mile. Avoid sketches that cover an area of several miles. In all cases, the termination point of the to-reach site must be on the sketch. The sketch does not need to be drawn to scale.
• Orientation. The sketch must be oriented to the north. DA Form 1959 has a preprinted arrow to indicate the direction.

3-55. The recon party will use transportation that is organic to the unit according to the unit's TOE and SOPs. When available, due to project requirements or customer support, using aircraft will enhance the project recon. Helicopters can greatly assist and speed recon efforts (for example, checking routes of travel and lines of sight between stations, selecting and identifying stations, and determining the scheme for extending surveying control). If aircraft are used, it is mandatory that the pilots be thoroughly briefed on the survey project. Complete knowledge of the entire project by the pilots will expedite the field recon and accelerate the progress of the project.

3-56. The recon party has access to radios, according to the unit's TOE and SOPs. Surveyors use the radios to confirm lines of sight when stations are separated by great distances. Before using the radios on a survey project, the party chief will obtain authorized frequencies from the local (customer's) signal officer. Surveyors will use the radios according to local CEOI and communications-electronics standing instruction (CESI). Surveyors will also follow the unit's standing signal instructions (SSI), signal operation instructions (SOI), and radio-communications procedures. In the event of a conflict, the procedures of the local signal office will take precedence.

3-57. The party chief will make arrangements with the customer to ensure that both fuel and maintenance are available for all vehicles. He will also ensure that adequate space is available to secure equipment and to perform project administration and field-office computing.

RECON REPORTS

3-58. Upon completion of the field recon, the party chief will submit a recon report. If the area or the project is large, the project will be divided into phases and a report will be prepared at the completion of each phase. The recon report is discussed in detail in Chapter 11.