6.3 Approved Calculator Operations & Memory Management (Casio fx-115/991, HP 35s, TI-36X)

6.3 Approved Calculator Operations & Memory Management

Speed, efficiency, and computational accuracy on the NSPS CST Level I examination depend heavily on calculator mastery. The CST Board strictly enforces calculator policies modeled after the National Council of Examiners for Engineering and Surveying (NCEES) guidelines. Survey technicians must know which calculator models are permitted, how to clear memory registers to comply with exam protocols, how to utilize specialized functions (such as DMS conversions and vector entry), and how to leverage memory registers to eliminate rounding errors during multi-step geometric operations.

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NSPS CST Calculator Policy & Approved Models

To maintain exam security and fairness, the NSPS CST Board prohibits any device capable of wireless communication, external text storage, or custom user programming that bypasses fundamental mathematical testing.

Permitted Calculator Series

Only the following non-programmable calculator models (and their specific sub-model variants) are allowed in the CST examination room:

1. Casio: fx-115 and fx-991 models (e.g., fx-115ES, fx-115ES Plus, fx-991EX ClassWiz, fx-991CW).
2. Hewlett-Packard: HP 33s and HP 35s models.
3. Texas Instruments: TI-30X and TI-36X models (e.g., TI-30Xa, TI-30X IIB, TI-30XS MultiView, TI-36X Pro).

Prohibited Features & Examination Rules

• Strictly Prohibited: Cell phones, tablets, smartwatches, laptops, TI-84/89/Nspire graphing calculators, HP Prime, and any calculator with a QWERTY keyboard.
• Covers and Hard Cases: Must be removed from the calculator and placed under the desk prior to testing.
• Proctor Inspection: Proctors inspect all calculators and mandate a full memory reset before test distribution.

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Clearing Calculator Memory Registers

Prior to starting the CST exam, candidates must clear all stored variables, statistical registers, and system memory. Performing memory resets quickly and confidently is an essential administrative skill.

+-------------------------------------------------------------------------+
|                    MANDATORY MEMORY RESET KEYSTROKES                    |
+-----------------------------------+-------------------------------------+|
| Calculator Series                 | Keystroke Sequence                  ||
+-----------------------------------+-------------------------------------+|
| Casio fx-115ES Plus / fx-991EX    | [SHIFT] -> [9] (CLR) -> [3] (All)   ||
|                                   | -> [=] (Yes) -> [AC]                ||
+-----------------------------------+-------------------------------------+|
| TI-36X Pro                        | [2nd] -> [reset] (above 0 key)      ||
|                                   | -> [2] (Yes) -> [enter]             ||
+-----------------------------------+-------------------------------------+|
| HP 35s (RPN / Algebraic)          | [CLEAR] -> [A] (ALL) -> [ENTER]     ||
|                                   | (or [LS] -> [C] CLEAR -> [ALL])     ||
+-----------------------------------+-------------------------------------+|

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RPN (Reverse Polish Notation) vs. Algebraic Entry

Approved calculators utilize two primary mathematical entry methods: Algebraic notation (Casio, Texas Instruments) and Reverse Polish Notation (RPN) (Hewlett-Packard).

Algebraic Mode (Infix Notation)

In algebraic entry, equations are entered in standard written order ($A + B =$). The calculator relies on internal rules of algebraic order of operations (PEMDAS). Complex fractions require parentheses to group numerators and denominators:

$\text{Keystrokes for } \frac{450.25 + 128.50}{32.10 - 14.50} \implies \text{( 450.25 + 128.50 ) / ( 32.10 - 14.50 ) =}$

RPN Mode (Postfix Stack Operations)

Developed by Hewlett-Packard, RPN places operators after numbers ($A$ ENTER $B$ +). Values are stored on an automatic 4-level operational stack consisting of registers X (bottom display), Y, Z, and T (top).

1. When you type a number and press ENTER, the value drops into Register X and pushes previous values up the stack ($X \to Y \to Z \to T$).
2. When you execute an operation (such as + or \times), the operation evaluates Registers X and Y immediately, displays the answer in Register X, and drops the stack down.

Comparison: Pythagorean Distance $HD = \sqrt{450.25^2 - 18.50^2}$

• Algebraic (TI-36X Pro): 2nd √ ( 4 5 0 . 2 5 x² - 1 8 . 5 0 x² ) enter (13 keystrokes)
• RPN (HP 35s): 4 5 0 . 2 5 x² ENTER 1 8 . 5 0 x² - √x (10 keystrokes, zero parentheses)

RPN Advantage: RPN eliminates parentheses entirely, displays intermediate results step-by-step, and drastically reduces keystroke errors during lengthy coordinate geometry calculations.

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Dedicated DMS Conversion Keys

Converting between Degrees-Minutes-Seconds ($D^\circ M' S''$) and Decimal Degrees ($DD$) is one of the most frequent operations on the exam. Each approved calculator features specific shortcut keys.

1. Casio fx-115ES Plus / fx-991EX

• Entering DMS: Use the dedicated [° ' "] key after each component.
  • Example ($45^\circ 12' 30''$): Type 45 [° ' "] 12 [° ' "] 30 [° ' "] =.
• DMS to DD: With the DMS angle displayed, press the [° ' "] key (or S<=>D). The display toggles to 45.20833333°.
• DD to DMS: Enter 45.20833333 = then press SHIFT $\to$ [° ' "]. Display converts to 45°12'30".

2. Texas Instruments TI-36X Pro

• Entering DMS: Access the Angle menu via 2nd $\to$ [angle] (above the apps key). Select 1: °, 2: ', and 3: ".
• DD to DMS Shortcut: Enter decimal degree value $\to$ Press 2nd $\to$ [angle] $\to$ Select 4: ▶DMS $ o$ Press enter.

3. HP 35s (RPN Mode)

• HMS Format: Angles in DMS are represented in HP display format as $D.MMSSss$ (e.g., $45^\circ 12' 30''$ is entered as 45.1230).
• DMS to DD: Type 45.1230 $\to$ Press LS (Left Shift - Blue) $ o$ [→DEG] (above the 8 key). Display shows 45.2083.
• DD to DMS: Type 45.208333 $\to$ Press LS $ o$ [→HMS] (above the 7 key). Display shows 45.1230.

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Memory Management & Avoiding Rounding Errors

Multi-step surveying problems (such as computing latitude, departure, and final coordinates) require chaining several mathematical operations together. Writing down intermediate results on scratch paper and re-keying rounded numbers introduces cumulative rounding error.

Storing and Recalling Intermediate Values

Approved calculators provide independent memory registers (A, B, C, D, E, F, X, Y, M).

• Casio fx-115/991: Press STO followed by the variable key (e.g., STO $\to$ A). To recall, press RCL $ o$ A.
• TI-36X Pro: Press the sto-> key, then toggle to the desired variable (x, y, z, t, a, b, c, d) and press enter. To recall, press 2nd $ o$ [recall].
• HP 35s: Press STO $ o$ variable letter key (e.g., STO $ o$ A). To recall, press RCL $ o$ A.

Golden Rules of Surveying Computation

1. Maintain Full Precision: Store unrounded intermediate values (full 10 to 12 decimal places) in memory registers.
2. Execute Operations in Memory: Recall stored values directly into trigonometric and algebraic functions.
3. Round Only at the Final Step: Round your final calculated answer to match the required precision (e.g., nearest 0.01 ft or nearest 1 arcsecond).