Navigational Information

Navigational Information

Navigational information consists of isogonic lines and values, local magnetic disturbance notes, aeronautical lights, airway intersection depictions, and VFR checkpoints. Isogonic lines (lines of equal magnetic declination for a given time) provide the pilot with the difference between true north and magnetic north in degrees. These lines and values are updated every five years. Local magnetic notes alert pilots to areas where the magnetic compass might be unreliable, often due to large deposits of iron ore. 

Aeronautical lights at one time were a primary means of navigation —the lighted airways of the 1930s. Light symbols are depicted on visual charts for their navigational value; however, because electronic aids have taken over the majority of navigational needs, many of the old and large airport beacons have been replaced with smaller units. This can lead to confusion. The Bakersfield, California, Meadows Airport has one of the small, less-intense beacons, while the nearby Shafter Airport has the large unit. Pilots, including Army helicopter pilots, regularly key on the Shafter beacon, and even land at the wrong airport. The larger lights can often be seen twice as far as the smaller units. Airport rotating or oscillating beacons are indicated by a star adjacent to the airport symbol and operate sunset to sunrise. Rotating lights (with flashing code identification) and course lights are from the lighted airway days, and almost all have been decommissioned.

Named intersections that can be used as reporting points are depicted on some visual charts. The intersections consist of a fiveletter name and might be difficult to pronounce. Use of intersections by VFR pilots is limited because the cross-fix radial headings are not shown. Intersections made up of VOR radials are shown in blue; those made up with low-frequency radio beacons are shown in magenta.

Visual ground signs and VFR checkpoints that are easily recognizable from the air are depicted. Many visual signs are left over from the early days of aviation, have faded, and are of little value; others are large and prominent.

Special conservation areas a20re shown on sectional charts. Landing, except in an emergency, is prohibited on lands or waters administered by the National Park Service, U.S. Fish and Wildlife Service, or U.S. Forest Service without authorization. All aircraft are requested to maintain a minimum altitude of 2000 feet above the surface. The Yosemite National Park is one of these areas.

Airspace

Airspace

Controlled airspace has become rather complicated and, ironically, it affects the VFR pilot to a much greater degree than the IFR pilot. The reason why airspace has become so complicated can be explained by its evolution. In the early days of aviation, all flying was visual; there was no such thing as controlled airspace or air traffic control. It wasn’t until the middle 1930s that blind flying using instruments became practical. Instrument flying came into its own after World War II with the development of navigational aids and communications.

The purpose of controlled airspace is to provide a safe environment for instrument operations. Controlled airspace established weather minimums. As one might expect, controlled airspace originally developed around airports where air traffic was congested. The next logical extension included the then-new electronic airway system.

As radios become more common, certain airspace required the pilot to establish radio communications with the controlling authority. As jet travel increased, all airspace in the contiguous United States above 14,500 feet became controlled, and flights above 24,000 feet required an IFR clearance. In the 1960s and 1970s, more and more airspace became controlled. Airspace that required a clearance for all aircraft was lowered to 18,000 feet, along with the airspace around major terminals. Specific communications and aircraft equipment requirements were established around smaller terminals.

The VFR pilot of the twenty-first century must contend with various weather minimums in an alphabet soup of controlled airspace, establish communications in certain areas, and make sure the aircraft has the required electronic equipment.

For our purposes, airspace on visual charts can be divided into two basic categories: controlled airspace and special-use airspace. Controlled airspace designated on visual charts is Class G, E, D, C, or B airspace. Their primary purpose is to protect IFR aircraft when weather conditions do not allow see-and-avoid separation. Class A airspace is not depicted. Special-use airspace is designated as prohibited, restricted, warning, alert, and military operations areas and military training routes. 

With the preceding as a background briefing of sorts, let’s see if we can make some sense out of the chaos. Areas with no air traffic control services are designated Class G airspace. Airspace not designated as Class E, D, C, or B on visual charts is Class G airspace. The Mammoth Lakes Airport is in Class G airspace.

If we were to fly west from Mammoth toward Merced, we would encounter a blue vignette encompassing airway Victor 230. (Note the number “115” in the box just below the airway designation. This is total mileage in nautical miles between NAVAIDs on the airway.) The blue vignette designates controlled Class E airspace at 1200 feet AGL. The dark edge of the vignette indicates the limit, and the vanishing edge is the direction of controlled airspace. Continuing west, we exit Class E airspace northwest of the airway.

When the base of Class E airspace is above 1200 AGL, the lower limit is printed on the chart. Continuing westbound, we come across another blue vignette (along the 119°30’ longitude line). The base of this Class E airspace is 12,000 feet MSL. 

The staggered thick-blue line (just east of Victor 165) indicates a change in the floor of Class E airspace. With no altitude specified, Class E airspace begins at 1,200 AGL.

From this point, we decide to fly direct to the El Nido VOR, near Merced. Class E airspace continues at 1200 AGL until we’re about two miles from the VOR, where we see a magenta vignette. The magenta vignette indicates that Class E airspace begins at 700 AGL.

If we were to proceed to the Merced Airport, we would encounter a magenta dashed line around the airport. The magenta dashed line means Class E airspace starts at the surface (surface-based Class E airspace). A magenta dashed line also designates surface-based Class E airspace associated with other types of airspace. For example, note the magenta dashed lines north of Castle AFB and south of Modesto.

At what altitude does Class E airspace begin over the Mammoth Airport? Unless designated at a lower altitude, Class E airspace begins at 14,500 MSL over the United States, except for airspace that is less than 1500 feet AGL (above mountains that are 13,000 or more feet MSL). (A sage pilot once observed that the first word in the Federal Aviation Regulations was “except.”) Class E airspace extends upward to but not including 18,000 feet.

The purpose of weather minimums is to allow enough ceiling, visibility, and cloud clearance for VFR and IFR aircraft to “see and avoid.” VFR weather minimums —that’s what they are, minimums— evolved in much the same way as controlled airspace. VFR weather minimums, especially below 10,000 feet, are much the same as they were in the beginning days of the Piper Cub and DC-3. With highperformance airplanes of all sizes and capabilities flying in the same airspace, a weather “minimum” does not necessarily equate to “safe”! 

As well as establishing weather minimums, Class D, C, B, and A airspace impose one or all of the following requirements:
• Communications
• AIRCRAFT EQUIPMENT
• ATC clearance
• Minimum pilot qualifications

Figure shows a vertical cross-section of the airspace with VFR minimums and certain equipment requirements.

All Class D airspace is surface-based. The upper limit is normally 2500 feet AGL. Modesto Class D airspace extends up to and includes 2600 feet MSL. This is indicated by the blue number “26” in the dashed blue box.

Class C airspace extends generally from the surface to 4000 feet AGL around airports with control towers and is served by a radar approach control. The boundaries of Class C airspace are individually tailored, based upon terrain and operational requirements. Class C airspace is charted using solid magenta lines. Various “shelves” exist beyond the surface-based airspace. Bases and tops of the “shelved airspace” are indicated in magenta (SFC/42 indicates “surface to 4200 feet MSL”; 14/42 indicates “1400 feet MSL to 4200 feet MSL”). Examine the Castle AFB Class C airspace.

Class B airspace surrounds the busiest airports. Class B generally consists of the airspace from the surface to 10,000 feet MSL with various shelves, sometimes referred to as an “upside-down wedding cake.” Class B airspace is charted using solid blue lines. Boundaries are defined by VOR radials, DME arcs, and prominent landmarks. Like Class C airspace, the bases and tops are charted. Like Class C airspace, bases and tops of Class B airspace are indicated, in this case in blue. 

Occasionally, Class D airspace extends to the base of overlying Class C or Class B airspace. This is one reason for nonstandard Class D airspace tops; in any case, the height is specified on the chart. When Class C airspace terminates at the base of Class B airspace, it is indicated by the magenta “T” (T/15 base of Class C 1500-feet upper limit base of overlying Class B airspace). The Oakland Class C airspace over the Hayward Airport. (The letter “T” is a leftover from the old airspace classification: the base of the terminal control area.) 

Class A airspace consists of that area from 18,000 to 60,000 feet MSL. Class E airspace is that area above 60,000 feet MSL. Class F is not an airspace designation in the United States; however, Class F is an International Civil Aviation Organization (ICAO) airspace classification. Where applicable, IFR and VFR flight are permitted. Air traffic advisory service and flight information service are provided on request. An ATC clearance is not required.

Fixed-wing special VFR is normally available in surface-based controlled airspace. In certain high-density surface-based airspace, special VFR is prohibited. This is indicated in the airport data block by “NO SVFR.”

Terminal radar service areas (TRSAs) designate airspace where traffic advisories, vectoring, sequencing, and separation of VFR aircraft are provided. TRSAs are designated Stage I, Stage II, or Stage III, which specify radar services that are available. The type of TRSA (Stage I, II, or III) can be found in the Airport/Facility Directory.

Visual charts depict special-use airspace (SUA) below 18,000 feet MSL. SUA consists of airspace where activities must be confined because they pose a hazard to aircraft operations. Prohibited, restricted, warning, alert, and military operations areas and military training routes are shown, with special military activity routes. Aircraft operations are prohibited within prohibited areas. These areas are established for security or other reasons associated with the national welfare. Aircraft operations are prohibited within restricted areas when the area is active. Restricted areas are established for unusual, often invisible, hazardous activities, such as artillery firing, aerial gunnery practice, or guided missile firing. Warning areas are established for the same hazards as restricted areas, but over international waters. Alert areas inform nonparticipating pilots of areas that might contain a high volume of training or unusual activity. Pilots should exercise extra caution within these areas.

Military operation areas (MOAs) alert pilots to military training activities. In addition to a possible high concentration of aircraft, military pilots might conduct aerobatic flight and operate at speeds in excess of 250 knots below 10,000 feet. High-speed low-level military operations are conducted along military training routes (MTRs). An MTR is designated IR when IFR operations are conducted within that route; VFR operations are designated VR. IR and VR routes operated at or below 1500 feet AGL will be identified by four-digit numbers (IR 1007, VR 1009). Operations that are conducted above 1500 feet AGL are identified by three-digit numbers (IR 205, VR 257). Special military activity routes alert pilots to areas where cruise missile tests are conducted.

Alert areas, like MOAs, advertise a high concentration of military activity. Figure shows alert area A-251 near Castle AFB, which warns pilots of military practice instrument approaches.

Figure contains symbols that alert pilots to parachute jumping, glider operations, and ultralight activity. An additional symbol has been added for hang-gliding activity. The symbol resembles a hang glider in flight. Where these symbols appear, pilots cannot expect to be alerted to the activity through NOTAMs. Details on the activity are normally found in the Airport/Facility Directory.

Radio Aids to Navigation

Radio Aids to Navigation

Figure shows standard symbols for the very high frequency omnidirectional radio range (VOR), a VOR collocated with distance measuring equipment (DME), and a VOR collocated with a tactical air navigation (TACAN) facility. A TACAN provides azimuth information similar to a VOR, but on an ultra high frequency (UHF) band used by military aircraft, and distance information from the DME. When the NAVAID is located on an airport, the type of facility (in this case VOR only) appears above the NAVAID box; otherwise, the appropriate symbol indicates the type of facility: VOR, VOR/DME, or collocated VOR and TACAN (VORTAC).

Note that the NAVAID box provides frequency and identification information for the Ontario VOR. The VOR frequency is 117.0 MHz, identification ONT, followed by a representation of its aural Morse code signal. The lower right corner shows a VORTAC NAVAID box. This is the Pomona VORTAC. The VOR frequency is 110.4 MHz, and the DME and TACAN channel (Ch) is 41. (Because VOR frequencies and DME channels are paired, when a pilot chooses the VOR frequency, the paired DME channel is automatically selected.) The identification of the NAVAID is POM, followed by the representation of the aural Morse code signal.

Locations were abbreviated with two letters in the early days of aviation, for instance NK was Newark. As the number of NAVAIDs and airports increased, three letter identifiers came into use. All VOR, VOR/DME, VORTAC, and many low-frequency radio beacons have three-letter identifiers.

Figure contains other standard NAVAID and flight service station communication symbols. Low- and medium-frequency NAVAIDs are shown in magenta; VOR, VOR/DME, and VORTACs are in blue. Low- and medium-frequency radio ranges have been decomissioned. Nondirectional beacons, marine beacons, and broadcast-station symbols are shown.

Heavy-line boxes indicate standard simplex FSS communication frequencies 121.5 and 122.2 MHz; simplex means one-way radio communications in which the pilot transmits and receives on the same channel. Other FSS frequencies are printed above the box —for example, 123.6 (for local airport advisories) and FSS discrete frequencies. Routine communications should be accomplished on the station’s discrete frequency. These frequencies are spread apart at individual facilities and locations to avoid frequency congestion with aircraft calling adjacent stations.

If a frequency is followed by the letter R (122.1R), the FSS has only receive capability on that frequency; therefore, the pilot transmits on 122.1 (or another designated frequency). The pilot must tune another frequency, usually the associated VOR, to receive voice communications from the FSS. This duplex communication requires the pilot to ensure that the volume is turned up on the VOR receiver. For example, in the upper right box of Fig. 11-12, the Prescott FSS has a receiver located at the Flagstaff VOR on 122.1R, noted above the NAVAID box. A pilot wishing to communicate through the VOR would tune the transmitter to 122.1 MHz, and select Flagstaff, 108.2 MHz, on the VOR receiver. An FSS can transmit on many frequencies (VORs and remote outlets, for instance). With FSS consolidation, it is important for the pilot to advise the FSS which frequency is being monitored in the airplane and the airplane’s general location. For example, “Reno Radio, Cessna four three three four echo, listening one two two point six, Ely, over.”

Note that only selected frequencies are depicted on these charts. Because en-route flight advisory service (flight watch) has a common frequency of 122.0 MHz, the frequency is not shown. Pilots calling flight watch should always include their approximate location on initial contact. Approach control and air route traffic control center frequencies are also omitted. Other frequencies are on the chart’s end panels and margins, in the Airport/Facility Directory, or are available from an FSS.

A small square in the lower right corner of the NAVAID box indicates hazardous in-flight weather advisory service (HIWAS) is available on the VOR frequency. The circled letter “T” means that a transcribed weather broadcast (TWEB) is transmitted over the VOR. An automated weather observation system and the frequency (AWOS-3 135.425) advertises the availability of this service. An underlined frequency indicates no voice communications available on that particular frequency.

Airport Aeronautical

Airport Aeronautical

Visual charts depict civil, military, and some private, landplane, helicopter, and seaplane airports. Hard-surfaced runways of 1500 to 8000 feet are enclosed within a circle depicting runway orientation. All recognizable runways, including some that might be closed, are shown for visual identification. Hard-surfaced runways greater than 8000 feet do not conveniently fit in a circle; the circle is omitted, but runway orientation is preserved. Airports with other than hard-surfaced runways, such as dirt, sod, gravel, and the like, are depicted as open circles. Airports served by an FAA control tower (CT) or non-federal control tower (NFCT) are shown in blue; all other airports are in magenta, which is a purplish red color. Tick marks around the basic airport symbol indicate the availability of fuel and that the airport is tended during normal working hours. Pilots should keep in mind that types of fuel and specific hours attended are contained in the Airport/Facility Directory, with changes or nonavailability of services mentioned in NOTAMs.

Restricted, private, and abandoned airports are shown for emergency or landmark purposes only. Pilots wishing to use restricted or private landing facilities must obtain permission from that airport authority. A check of your insurance policy might also be in order. Some insurance policies restrict landings to public airports, except in emergencies. Airports are labeled unverified when available for public use, but warranting more than ordinary precautions due to lack of current information on field conditions, or available information indicates peculiar operating limitations. Selected ultralight flight parks appear only on sectional charts as an “F” within the airport circle.

Figure 11-10 decodes standard airport information. The circled letter “R” preceding the airport name indicates the availability of airport surveillance radar, and the airport location identifier follows the airport name: R Oakland (OAK). With airspace reclassification, Class D airspace replaced the control zone and airport traffic area. This all but eliminated the need for special airport traffic areas defined by FAR 93. Special airport traffic areas are indicated on the chart by the airport name placed within a box, for example, Anchorage. Two still exist in Alaska at Anchorage and Ketchikan. Specific requirements are contained in the Alaska Supplement, Regulatory Notices.

FSS indicates a flight service station on the field, and RFSS indicates a remote flight service station. These facilities might provide a local airport advisory at selected airports. The decision whether AFSSs will provide local airport advisories is still in question. Only the primary tower local control frequency appears. A star following the local control frequency indicates a part-time tower. Supplemental and additional frequencies, such as approach, secondary local control and ground frequencies, and tower hours of operation are contained on the end panels or margin of the chart and in the Airport/Facility Directory. Automatic terminal information service (ATIS), where available, is always shown. Supplemental frequencies, such as an aeronautical advisory station (UNICOM) or VFR advisory service, might be listed. The letter “C” within a circle indicates the common traffic advisory frequency (CTAF): not shown on WACs. This frequency is usually the tower frequency at airports with part-time towers. Airport elevation is always in feet above mean sea level and never abbreviated, 285 feet MSL.

Runway length is the length of the longest active runway, including displaced threshold and excluding overruns. Runway length is shown to the nearest 100 feet, using 70 as the division point; a runway 8070 feet long is charted as “81,” and a runway 8069 feet long is charted as “80.” In the example, the longest runway is 7200 feet.

Airport lighting, indicated by the letter “L,” operates sunset to sunrise, unless preceded by an asterisk, which indicates limitations exist. All lighting codes refer to runway lights. The lighted runway might not be the longest or lighted full length. Pilots must refer to the Airport/Facility Directory for specific limitations, such as pilotcontrolled lighting. Other remarks are added as required, such as airport of entry. When information is not available, the respective character is replaced by a dash.

Culture

Culture

Constructed land features include roads and highways, railroads, buildings, canals, dams, boundary lines, and the like. Many landmarks that can be easily recognized from the air, such as stadiums, racetracks, pumping stations, and refineries, are identified by brief descriptions adjacent to a small black square or circle marking exact location. Depictions might be exaggerated for improved legibility.

Figure contains a description of railroads, roads, bridges, and tunnels shown on aeronautical charts. Differences between sectionals and WACs are noted. Single-track railroads have one crosshatch; double and multiple railroads have a double crosshatch. Railroads often make excellent checkpoints. A word of caution. Numerous railroads emanate like spokes from many large cities. Pilots navigating exclusively by the “iron compass” have become hopelessly confused when they inadvertently took the wrong track —pardon the pun.

Never navigate solely by one landmark. Major highways (category 1) also make excellent checkpoints, but they do suffer from the same problems as the railroad. Secondary roads (category 2, and especially secondary category 2) are often difficult to positively identify, especially when flying over sparse areas of desert or plains. Bridges, viaducts, and causeways are often very good checkpoints.

Figure shows populated areas (large cities from figure), boundaries, water features, and miscellaneous cultural features. Large and medium cities are shown by their outlines as they appear on the ground. This helps significantly with identification. Towns and villages are only represented by a small circle. Especially where several towns or villages are in the same general area, this symbology makes them hard to positively identify. Political boundaries are shown using standard map symbols. Cultural coastal features are depicted because of their landmark value. Small mines and quarries are shown by a small crossed-picks symbol.

Pilots should pay particular attention to the symbol for aerial cableways, conveyers, and the like, which are formally called catenaries. The catenaries depicted on aeronautical charts are cables, power lines, cable cars, or similar structures suspended between Pilots should pay particular attention to the symbol for aerial cableways, conveyers, and the like, which are formally called catenaries. The catenaries depicted on aeronautical charts are cables, power lines, cable cars, or similar structures suspended between peaks, a peak and valley below, or across a canyon or pass. A cableway is normally 200 feet or higher above terrain, which poses a very serious hazard to low-flying aircraft; the cable might be marked with orange balls or lights.

Cultural features are not revised as often as aeronautical information; therefore, especially in areas of rapid metropolitan development, cultural features as seen from the air might differ from those depicted on the chart. Power transmission lines (high-tension lines) are depicted for their landmark and safety value. Often, transmission lines can be used to verify the identification of other landmarks. Although not normally qualifying as an obstruction, their depiction alerts pilots flying at low altitudes to this sometimes almost invisible hazard. Transmission lines are shown on a chart as small black towers connected by a single line.

Hydrography

Hydrography

Hydrography pertains to water and drainage features. Hydrographic features on aeronautical charts are represented in blue: Streams, rivers, or aqueducts are depicted by single blue lines; lakes and reservoirs are depicted by a blue tint. Small dots or “hatching” indicate where streams and lakes fan out (or are not perennial) or where reservoirs are under construction.

Figure shows how shorelines, lakes, streams, reservoirs, and aqueducts are depicted. Shorelines usually make excellent checkpoints, except where they are relatively straight without features. Pilots need to pay attention to shoreline orientation. For example, most people assume that California’s coastline is north-south; however, in certain areas, such as around Santa Barbara, the coast is actually east-west. This has led to much confusion for student pilots and others unfamiliar with the area. Lakes usually make good checkpoints, especially when their shape is unique or they are dammed.

Caution needs to be exercised with all lakes, perennial and nonperennial. A perennial lake contains water year round; a nonperennial lake is intermittently dry, usually during the dry season. There can be confusion during periods of drought when perennial lakes will be dry. Other discrepancies result from human decisions to drain, expand, or abandon reservoirs. If at all possible, streams should only be used to support other checkpoints. That is, there should be other landmarks that establish position that are supported by the position of the stream. Perennial and nonperennial streams should be treated with the same cautions as perennial and nonperennial lakes. Reservoirs are similar to lakes and can be treated in the same way; however, reservoirs are usually perennial.

Figure describes other hydrographical features contained on aeronautical charts. Among them are symbols for swamps, marshes, and bogs. A swamp is nothing more than a lake with trees growing out of it—an emergency landing there could be catastrophic. Pilots flying over unfamiliar terrain would be well advised to seek the advice of local pilots or the FSS that is responsible for that region.

Tundra describes a rolling, treeless, often marshy plain, usually associated with arctic regions. Hummocks and ridges describe a wooded tract of land that rises above an adjacent marsh or swamp. Mangroves are any of a number of evergreen shrubs and trees growing in marshy and coastal tropical areas; a nipa is a palm tree indigenous to these areas. Bogs are areas of moist, soggy ground, usually over deposits of peat. Flumes, penstocks, and similar features depict water channels used to carry water as a source of power, such as a waterwheel. Pilots flying in northwestern Montana and especially Alaska can expect to see glaciers and glacial moraines (debris carried by the glacier), ice cliffs, snow and ice fields, and ice caps. Other than canals, the other features in figure might be difficult to verify and should normally only be used to support other checkpoint features.

Figure depicts the remaining hydrographical features contained on aeronautical charts. Ice peaks, polar ice, and pack ice are features restricted to polar and arctic regions. Boulders, wrecks, reefs, and underwater features are displayed because they have certain landmark value. Some of these features might be small and difficult to identify.

One other topographical feature should be mentioned. Mountain passes are depicted by black curved lines outlining the pass. The name of the pass and its elevation are shown. Tioga Pass in figure, upper right, is shown with an elevation of 9943 feet MSL.

Terrain

Terrain

Three methods are used on aeronautical charts to display relief: contour lines, shaded relief, and color tints. Contour lines, as the name implies, connect points of equal elevation above mean sea level (MSL) on the Earth’s surface. Contours graphically depict terrain and are the principal means used to show the shape and elevation of the surface. Contours are depicted by continuous lines—except where elevations are approximate, then with broken lines—labeled in feet MSL. On sectional charts, basic contours are spaced at 500-foot intervals, although intermediate contours might be shown at 250-foot intervals in moderately level or gently rolling areas. Occasionally, auxiliary contours portray smaller relief features at 50-, 100-, 125-, or 150-foot intervals.

Figure shows how contours, shaded relief, and color tints depict terrain. Contours show the direction of the slope, gradient, and elevation. For example, in figure, valley floors have little or no gradient, while the mountains have steep gradients. The contours are labeled with their elevation. Shaded relief depicts how terrain might appear from the air. The cartographer shades the areas that would appear in shadow if illuminated from the northwest. Shaded relief enhances and supplements contours by drawing attention to canyons and mountain ridges. Color tints depict bands of elevation. These colors range from light green for the lowest elevations to dark brown for higher elevations. Color tints in figure range from light green in the valley to dark brown over the mountain ranges, supplementing the contours and enhancing recognition of rapidly rising terrain.

In addition to contours, shading, and tints, significant elevations are depicted as spot elevations, critical elevations, and maximum elevation figures. Spot elevations represent a point on the chart where elevation is noted. They usually indicate the highest point on a ridge or mountain range. A solid dot depicts the exact location when known. An “x” denotes approximate elevations; where elevation is known, but location approximate, only the elevation appears, without the dot or “x” symbol. Critical elevation is the highest elevation in any group of related and more-or-less similar relief formations. Critical elevations are depicted by larger elevation numerals and dots than are used for spot elevations. Figure illustrates the difference between spot and critical elevations.

Maximum elevation figures (MEF) represent the highest elevation, including terrain and other vertical obstacles —natural and constructed— bounded by the ticked lines of the latitude/longitude grid on the chart. Depicted to the nearest 100-foot value, the last two digits of the number are omitted. The center of the grid in the upper right portion of figure shows 128. The MEF for this grid is 12,800 feet MSL. This figure is determined from the highest elevation or obstacle, corrected upward for any possible vertical error (including the addition of 200 feet for any natural or man-made obstacle not portrayed), then rounded upward to the next higher hundred-foot level; therefore, almost all MEFs will be higher than any elevation or obstacle portrayed within the grid on the chart. Pilots should note that these figures cannot take into account altimeter errors and should be considered as any other terrain elevation figure for flight-planning purposes.

Latitude and longitude are labeled in degrees. Lines of latitude and longitude are subdivided by lines representing 10 minutes, and half lines representing 1 minute of arc. Because longitude represents the same distance anywhere on the Earth—unlike latitude, which decreases toward the poles—one minute of longitude anywhere on the Earth equals one nautical mile (nm); therefore, lines of longitude can be used for quick estimates of distance.

After large earthquakes in southern California in 1971 and 1994, perhaps the Los Angeles sectional chart should have contained the comment: “CAUTION—Terrain elevations subject to change without notice.”

Other topographical relief features considered suitable for navigation are contained in figure. They include lava flows, sand and gravel areas, rock strata and quarries, mines, craters, and other relief information usable for visual checkpoints.

Lava flows, sand ridges, and sand dunes are quite pronounced when seen from the air, and they make excellent checkpoints, especially if they are isolated by other terrain features. Unfortunately, most of these features only appear in the western United States. Strip mines and large quarries also make excellent checkpoints because of their visibility. Large craters, where they appear, also make excellent checkpoints.