IS 1491:1959 prismatic compass ke liye sabse purana aur mazboot standard hai jo aaj bhi survey labs aur field works mein follow kiya jata hai. Ye code 1959 mein banaya gaya tha aur instruments ki manufacturing quality ko control karta hai.

Is code ki poori detail technical point-of-view se niche di gayi hai:

IS 1491:1959 – Specification for Prismatic Compass

1. Scope (Kiske liye hai?)

Ye standard 85 mm, 100 mm aur 110 mm nominal size wale prismatic compass ke liye specifications deta hai. Ismein Liquid type (A) aur Dry type (B) dono shamil hain.

2. Construction & Design (Banawat)

• Non-Magnetic Body: Compass ki body aisi metals (Aluminium ya Brass) se banni honi chahiye jo magnetic needle ko disturb na karein.

• Prism: Prism high-quality optical glass ka hona chahiye jiske upar ek "Sun-shade" (Dark glasses) laga ho taaki dhoop mein bhi reading dikhe.

• Pivot: Pivot hamesha "Hardened Steel" ya "Iridium" ka hona chahiye taaki needle bina ruke smoothly ghum sake.

3. Graduated Ring (Graduation)

• Anti-Clockwise Marking: IS 1491 ke mutabiq, prismatic compass mein readings hamesha South se shuru ($0^\circ$) hoti hain aur clockwise direction mein badhti hain (West $90^\circ$, North $180^\circ$, East $270^\circ$).

• Inverted Figures: Ring par numbers ulte (Inverted) likhe hote hain taaki prism se dekhne par wo humein seedhe aur saaf dikhein.

4. Accuracy & Sensitivity

• Least Count: Is code ke mutabiq compass itna accurate hona chahiye ki hum 30 minutes ($30'$) tak ki reading bina kisi confusion ke le sakein.

• Error Limits: Needle hamesha Magnetic North se align honi chahiye. Agar error $30'$ se zyada hai, toh wo instrument IS standard ke mutabiq fail mana jata hai.

5. Liquid vs Non-Liquid (Type A vs Type B)

• Type A (Liquid): Ismein "Alcohol" aur "Water" ka mix ya "Purified Kerosene" bhara hota hai. Ye needle ke vibrations ko turant rok deta hai (Damping), jisse reading jaldi milti hai.

• Type B (Dry): Ismein koi liquid nahi hota. Needle ko haath se ya "Brake Pin" se rokna padta hai.

6. Major Components as per IS 1491

1. Lifting Pin: Jab hum compass band karte hain, toh lifting pin needle ko pivot se upar utha deti hai taaki pivot ghisne se bach jaye.

2. Agate Cap: Needle ke niche ek pathar (Agate) laga hota hai jo frictionless movement deta hai.

3. Vanes: Sighting vane mein "Horsehair" (ya fine wire) hona chahiye taaki object ko accurately bisect kiya ja sake.

NTS Study (Nodal Theory) Pro Tip:

Site engineers ke liye ye code isliye zaroori hai kyunki agar aapka compass IS 1491:1959 ke mutabiq hai, toh iska matlab hai ki uski Agate Cap aur Pivot itne majboot hain ki wo kayi saalo tak accurate reading denge.

Surveying Interview Question: "Prismatic compass mein zero ($0^\circ$) South par kyu hota hai?"

Answer: Kyunki prism hamesha observer ki side (South) hota hai aur reading uske niche se dekhi jati hai, isliye alignment ke liye zero South par rakha jata hai (IS 1491 standard).

Surveying Advanced: Top 101-200 One-Linear Q&A

Section 1: Errors & Measurements (Advanced)

101. Most probable value kya hoti hai? – Wo value jisme errors ke chances sabse kam hote hain (Arithmetic Mean).

102. Weight of an observation ka kya matlab hai? – Measurement ki reliability; ye error ke square ke inversely proportional hota ($W \propto 1/e^2$).

103. Accidental errors kis law ko follow karte hain? – Theory of Probability (Law of Errors).

104. Discrepancy kise kehte hain? – Ek hi quantity ke do measured values ke beech ka difference.

105. Systematic errors ko eliminate kaise karte hain? – Proper calculations aur corrections apply karke.

106. Tape standardization kyu zaroori hai? – Kyunki thermal expansion se tape ki asli length badal jati hai.

107. Tension correction kaisa hota hai? – Agar applied pull, standard pull se zyada hai toh Positive ($+$), warna Negative ($-$).

108. Normal Tension kise kehte hain? – Wo pull jahan Pull Correction aur Sag Correction ek dusre ko cancel kar dein.

109. Link ki length kaise measure karte hain? – Ek link ke center se agle link ke center tak.

110. Chain ki total length kahan se kahan tak hoti hai? – Dono handles ke outer ends tak.

Section 2: Compass & Traversing (Deep Analysis)

111. Magnetic Meridian samay ke saath kyu badalta hai? – Earth ke magnetic field mein badlav (Magnetic Storms) ki wajah se.

112. Secular Variation kya hai? – Wo magnetic variation jo sadiyon (centuries) mein hota hai.

113. Diurnal Variation kab zyada hota hai? – Din mein raat ke muqable aur Garmiyon mein Sardiyon ke muqable.

114. Local Attraction pehchanne ka sabse bada rule? – FB aur BB ka difference $180^\circ$ nahi hona.

115. Agar station affected hai, toh kya 'Included Angle' sahi hoga? – Haan, kyunki error dono lines par barabar hoga aur minus karne par cancel ho jayega.

116. Gale’s Traverse Table ka use kya hai? – Traverse calculations aur closing error ko check karne ke liye.

117. Omitted Measurements kya hain? – Jab field mein kuch lengths ya angles miss ho jayein toh unhe mathematically nikalna.

118. Closed Traverse mein angles ka sum kitna hota hai? – $(2n - 4) \times 90^\circ$ (Interior angles).

119. Exterior angles ka sum kitna hota hai? – $(2n + 4) \times 90^\circ$.

120. Deflection angle ki range kya hoti hai? – $0^\circ$ to $180^\circ$.

Section 3: Levelling & Contouring (Technical)

121. Sensitiveness of bubble tube kaise badhti hai? – Tube ka radius badhane se aur liquid ki viscosity ghatane se.

122. Leveling staff mein minimum reading kya hoti hai? – $0.005$ m ($5$ mm).

123. Parallax error kaise hatate hain? – Objective aur Eyepiece ko properly focus karke.

124. Inverted Staff reading kab li jati hai? – Jab point instrument station se upar ho (jaise Chhat ya Bridge ka niche ka hissa).

125. Change Point (CP) par kaunsi readings li jati hain? – Dono, FS (purana setup) aur BS (naya setup).

126. Profile Levelling kiske liye ki jati hai? – Road, Railway ya Canal ke longitudinal section ke liye.

127. Cross-sectioning kyu karte hain? – Earthwork volume calculate karne ke liye.

128. Grid Contouring kab use hoti hai? – Chote aur flat areas ke liye.

129. Radial Contouring kahan use hoti hai? – Pahadi (Hilly) ilakon mein.

130. Water surface (talab) ka contour kaisa hota hai? – Ekdum horizontal aur closed.

Section 4: Theodolite & Modern Tools

131. Double sighting kyu ki jati hai? – Instrumental errors (jaise non-perpendicularity) ko remove karne ke liye.

132. Vertical Circle ki reading kab zero hoti hai? – Jab telescope ekdum horizontal ho.

133. Index Error kya hai? – Jab vertical circle horizontal line par zero reading nahi deta.

134. Optical Square kis principle par kaam karta hai? – Double Reflection.

135. Total Station mein 'EDM' ka kya kaam hai? – Electronic Distance Measurement (infra-red rays ka use karke).

136. Prism ka role Total Station mein? – Signal ko wapas reflect karna.

137. Remote Elevation Measurement (REM) kya hai? – Bina point छुए uski height nikalna (jaise Bijli ke taar).

138. Remote Object Programming (ROP) kya hai? – Do points ke beech ka distance nikalna jahan pahunch pana mushkil ho.

139. GPS mein 'Segments' kya hain? – Space, Control, aur User segments.

140. DGPS (Differential GPS) kyu use hota hai? – Meter se centimeter tak ki accuracy paane ke liye.

Section 5: Curves & Tacheometry (Complex)

141. Simple Curve ka tangent point kise kehte hain? – Jahan straight line curve se milti hai (PC - Point of Curve).

142. Point of Tangency (PT) kya hai? – Jahan curve khatam ho kar wapas straight line banti hai.

143. Long chord ki length kab radius ke barabar hoti hai? – Jab deflection angle $60^\circ$ ho.

144. Rankine’s Method kiske liye use hota hai? – Curve setting by tangential angles (Deflection angle method).

145. Super-elevation ($e$) ka basic formula? – $e = v^2 / gR$.

146. Transition curve ki length kis par depend karti hai? – Rate of change of centrifugal acceleration par.

147. Vertical curves hamesha kaise hote hain? – Parabolic.

148. Summit curve kab banta hai? – Jab gradient upar ki taraf badhta hai.

149. Valley curve kyu diya jata hai? – Headlight sight distance aur comfort ke liye.

150. Tacheometry mein 'Stadia constant' kyu check karte hain? – Calculation ki accuracy ensure karne ke liye.

     Section 6: Plane Table Surveying (Instruments & Methods)

     151. Plane Table ka main principle kya hai? – Parallelism (Table har station par pehle station ke parallel honi chahiye).

     152. Alidade ka primary use kya hai? – Objects ko sight karna aur unki ray draw karna.

     153. Plain Alidade aur Telescopic Alidade mein kya farq hai? – Telescopic alidade se inclined sights aur badi distances accurately li ja sakti hain.

     154. Plumbing Fork (U-frame) kyu use hota hai? – Ground point ko table par accurately center karne ke liye.

     155. Trough Compass ka kaam? – Magnetic North line mark karna.

     156. Orientation ka sabse accurate method? – Back Sighting method.

     157. Magnetic Needle se orientation kab karte hain? – Jab accuracy kam chahiye ho ya stations door hon.

     158. Two-Point Problem kya hai? – Do pehle se pata points ki madad se naye station ki position fix karna.

     159. Three-Point Problem ka sabse fast method? – Tracing Paper method (Mechanical method).

     160. Bessel’s Method kya hai? – Three-point problem solve karne ka Graphical method.

     Section 7: Hydrographic Survey (Water bodies)

     161. Sounding ka kya matlab hai? – Pani ke niche depth measure karna.

     162. Fathometer (Echo Sounder) kis principle par kaam karta hai? – Sound waves ka reflection (Time interval measure karke).

     163. Sounding Rod kab use hoti hai? – Kam gehre pani ($< 5$m) ke liye.

     164. Lead Line mein weight kiska hota hai? – Lead (Sheesha) ka, jo rope ke end par bandha hota hai.

     165. Tide Gauge kyu use hota hai? – Pani ke level mein hone wale badlav (Tides) ko record karne ke liye.

     166. Mean Sea Level (MSL) kitne saalon ka average hota hai? – 19 saalon ka.

     167. India mein MSL kahan se liya jata hai? – Mumbai High (pehle Karachi tha).

     168. Shoreline Survey kyu kiya jata hai? – Nadi ya Samundar ke kinare ki boundary fix karne ke liye.

     169. Three-Point Resection hydrography mein kyu zaroori hai? – Chalti hui naav (boat) ki position fix karne ke liye.

     170. Sextant instrument ka use? – Naav se do shore objects ke beech ka horizontal angle napne ke liye.

     Section 8: Photogrammetry (Aerial Survey)

     171. Nadir Point kya hai? – Camera lens ke theek niche ground par point (vertical line).

     172. Isocenter kya hai? – Jahan tilt displacement zero hota hai.

     173. Crab (केकड़ा चाल) kyu hota hai? – Jab aeroplane ki axis flight line ke parallel nahi hoti (hawa ki wajah se).

     174. Drift kya hai? – Aeroplane ka apni asli flight path se side mein bhatak jana.

     175. Overlapping kyu zaroori hai? – 3D view (Stereoscopic vision) paane ke liye.

     176. Longitudinal Overlap kitna hona chahiye? – 60% (Commonly).

     177. Side Overlap (Lateral) kitna rakhte hain? – 30% (Commonly).

     178. Scale of Vertical Photograph? – $f / (H - h)$ ($f = \text{focal length}, H = \text{flight height}, h = \text{ground elevation}$).

     179. Stereoscope ka use? – Do overlapping photos ko 3D mein dekhne ke liye.

     180. Parallax bar kyu use hota hai? – Photos se points ki elevation nikalne ke liye.

     Section 9: Astronomy & Modern Surveying

     181. Celestial Sphere kya hai? – Ek imaginary sphere jiska radius infinite hai aur Earth center par hai.

     182. Zenith kya hai? – Observer ke theek upar celestial sphere par point.

     183. Nadir (Astronomy)? – Zenith ke theek opposite (theek niche) waala point.

     184. Azimuth kya hai? – Observer ke meridian aur star ke beech ka horizontal angle.

     185. Hour Angle? – Meridian aur star ke beech ka angle jo West ki taraf napa jaye.

     186. EDM (Electronic Distance Measurement) mein kaunsi waves use hoti hain? – Light waves, Infrared, ya Microwaves.

     187. Tellurometer mein kaunsi waves hoti hain? – Microwaves.

     188. Geodimeter mein kya use hota hai? – Visible light.

     189. Total Station mein data kaise save hota hai? – Electronic Data Logger ya Microprocessor mein.

     190. GIS (Geographic Information System) kya hai? – Data ko capture, store aur analyze karne waala computer system.

     Section 10: Survey Laws, Errors & Ethics

     191. Theory of Least Squares kab apply hoti hai? – Jab measurements mein sirf accidental errors hon.

     192. Law of Accidental Errors kya kehta hai? – Chote errors hone ke chances bade errors se zyada hote hain.

     193. Error of Closure ki limit? – Ye survey ke type par depend karta hai (e.g., $1/1000$ for rough work).

     194. Legal Survey (Cadastral Survey) ka maksad? – Property lines aur land ownership fix karna.

     195. Field Note book kaise bharni chahiye? – Pencil se, bina kisi overwriting ke (Direct entry).

     196. Surveyor ka ethical kaam? – Data ko kabhi "Cook" (fake entry) nahi karna chahiye.

     197. Mistake vs Error? – Mistake laparwahi se hoti hai (jaise galat digit padhna), Error limit se bahar ki cheez hai.

     198. Double Centering kyu karte hain? – Instrument ke vertical plane aur horizontal axis ke errors hatane ke liye.

     199. Standard Deviation ($\sigma$)? – Precision measure karne ka tarika; jitna kam $ \sigma $, utni zyada precision.

     200. Map Revision kyu zaroori hai? – Time ke saath ground features (roads, buildings) badal jate hain.