Green Slope

With the advent of Aimpoint and other measured styles of putting, it has become necessary to assess green slopes during practice to help calibrate and develop technique. Some golfers use expensive digital levels to make such measurements but using the "Green Slope" App you can measure slopes using your phone to a reasonable level of accuracy.

Using Green Slope

Putt Assessment

• Stand about half way to the pin.

• Press "Pin" or "Ball" button to select pin direction.

• Place phone across the putt line.

• Leave for about three seconds.

• Pick up when flashes or reaches high quality.

• Reset button can restart an assessment.

Preferences And Settings

• Select Facedown or Faceup mode.

• Set "Stimp" to an estimate of green speed.

• Set "Past" to your preferred 'past hole distance'."

• Choose Break from "Pin" or "Hole Edge" (Menu).

• Choose slopes in Percent or Degrees. 

• Calibrate Faceup mode, if using (Menu). 

Putt Break.

Putt break values shown in the tables are calculated in the app by applying typical ball motion physics on a sloping green. These are the distances from the pin or edge of the hole (by preference), perpendicular to the line between the ball and the pin, to the "aim point". The line from the ball to the aim point is the starting direction for the ball to ensure a putt at the right speed will fall into the hole.

Higher green speeds and smaller "distance past the hole" values will result in larger aim point breaks.

Aim Point is not the Apex of the Putt

This can't be overemphasized. The provided aim point is not the apex of the putt. Many golfers often assess putts from experience, with an estimate of break based on the apex of the putt, and consequently are likely to instinctively putt above the aim point line when using values from the table.

It can take some practice to get used to the concept of starting toward the aim point. From a technical standpoint, it is the tangent line to the putt curve at the start of the putt, projected onto the line perpendicular to the hole. If you select the break value for your putt distance in the break table and then select the graph button, the app will show you what the putt looks like from above, with it's y axis as the fall line.

Facedown or Faceup

With 'Green Slope', you can read the slope of greens using your iPhone faceup or facedown, depending on which side of your phone is flatest. Since most phones now have larger cameras sticking out on the back and are flatter on the front, 'Green Slope' has been developed to also allow you to place your device facedown 'across' the line to the hole, leave it sitting for a few seconds, and when you pick it up the display will show the putt line slopes. 

If preferred, pressing the 'Facedown' button at the top of the display selects the 'Faceup' mode, allowing slope measurement faceup with the phone is sitting on the ground. To use this mode effectively, you made need to calibrate the faceup mode (from the menu) to correct for any camera bumps on the rear. 

Slope Measurement Quality

The slope arrows and text will change color to show the estimated quality of the measurements. Blue indicates the highest quality, Orange is medium quality and Red suggests potentially low quality. 

Quality is assessed from the variance of the data with sufficient high quality points to ensure measurements are meaningful. Low or medium quality may result from movement of the phone while making a measurement. The app takes many measurements over about 3 seconds, but waits until high quality is achieved before stopping and locking its assessment.

• Red Low Quality

• Orange Medium Quality

• Blue High Quality

Display Arrows

Once a high quality measurement has been completed and slopes have been locked, the arrows on the display are continuously adjusted to show directions even while you move around.

Pin Arrow

A blue arrow shows the direction to the pin. If you stand behind the ball, with your phone pointing at the pin, the "Pin Arrow" should point at the pin. If it's pointing at you, press the "Ball button to reverse the pin arrow. Setting this is important as it's used to decide whether the putt break is uphill or downhill relative to the hole.

Fall Line Arrow

The green 'Fall Line' is the maximum slope of the green surface at the location of the measurement. It's the direction that a ball would eventually fall if simply pushed down the hill. This line is sometimes referred to as the Gravity Vector since gravity acts on the ball in this direction.

Clock Angle

The relationship of the putt line to fall line is the putt clock angle. The latter together with green speed and selected distance past the hole, determine the break on the putt. Clock angle is measured from the fall line down. A clock angle of 90 degrees is a putt that is perpendicular to the fall line. Anything less than 90 degrees is a downhill putt and anything greater than 90 degrees is an uphill putt. Since actions are simply mirrored on either side of the fall line, clock angle is always in the 0 to 180 degree range.

Green Speed

Green speed is measured using a USGA approved "Stimp Meter". This is a aluminum channel that is lifted to an inclination of 20 degrees, that causes a ball to roll onto the green rolling at a standard speed of 6ft/sec. Grass friction slows the ball after it rolls off the slope, and the distance in feet from the end of the Stimp meter to where the ball comes to rest is a measure of the "green speed" or "Stimp" value. To measure accurately, it will be evaluated using three balls in one direction, and then measured again in the opposite direction, with the final Stimp value being an average of all results in feet. 

Each golf course decides on its green speed based a number of factors it can be anything from about 5ft to 15ft. Higher Stimp values mean faster greens.

On most golf courses you can get an estimate of likely green speed or Stimp from the Proshop.

Adjusting Green Speed

Once you have a likely estimate for the green speed, you can often improve this on the practise green by choosing an area with a reasonably consistent slope of at least 2% and then making some putts from maybe 15ft and measuring the resulting break relative to the aim point as described above. Compare with the Green Slope App's estimated break, and then adjust the Stimp to match the actual putt breaks.

Standard break table.

Standard break tables are available from the menu. These help understand the aim point variations with green slope and putt distance. These can also be viewed in a graph from the same menu.

Selecting a specific break value in the table and then pressing the graph button will also show an overhead view of the likely putt trajectory for any specific putt.

Accuracy And Repeatability

This App uses all of the measurement sensors in your device to assess green slopes relative to the surface of the device. Realize that a 1% slope is a one inch increase over a hundred inches (~8ft) and 0.1% is only a tenth of an inch over the same distance! This is one of the reasons that green reading can be hard. With a 5 inch long phone, 1% is 0.05 inches and 0.1% is only 0.005 inches, and the grass on greens is typically cut to a height of about 0.125 inches! However, measuring slope with your phone is sufficiently accurate if it is done with care.

Repeatability is difficult to assess on real greens as small movements of the phone on with the roughness of the cut grass will affect results. Ensuring that you are using the flatest surface of your device will make measurements more reliable. Device sensors will also have some degree of drift due to the physics their small size. Taking measurements over a longer period helps reduce the drift effect.

However, experience and testing have demonstrated that slopes can typically be read to an accuracy of better than 0.3% in most situations. Slopes less than 0.5% should be reviewed carefully as the green is nearly flat and small variations on the putt line can affect the outcome.

USGA Rules

Note that measuring slope using an App is not approved under the USGA rules of golf and this should only be used for practice.

App Store: Green Slope.