Hello from Cincinnati, Ohio

[edfrank]

Usually the top is at a pretty steep angle so a difference of 1/2 yard will make a difference in tree height. The angle to the base is commonly close to horizontal. A distance to the base that is off by 1/2 yard, will make only a minimal difference in the tree height totals.

At 50 yards at an angle or 45 degrees the maximum error of 1/2 yard will result in a height difference of 12.7 inches.

At 50 yards at an angle of 5 degrees the maximum error of 1/2 yard will result in a height difference of 1.5 inches.

Actual errors would be some fraction of this amount. So if you move horizontally to a click-over point for the lower reading, the unintended variation in level caused by moving is likely greater than the error from staying at the some point as the upper click-over point, particularly for shallow angles. To summarize, if the angle to the base is small you are probably better off not moving to a click-over point.

Ed Frank

[Matt Markworth]

Cool, makes sense.

I plan on calibrating the laser rangefinder tomorrow. I'll notate the click-over point every 5 yards and see what variations I get.

Thanks,
Matt

[Matt Markworth]

Here are the results of calibrating my Nikon Prostaff 440.

Conditions: Clear Day, Sun directly behind me
Target: White label

Based on these findings, I’ll be reducing the distance by the following amounts when I back up and find the click-over point when shooting to a highly reflective target. I can also calibrate using a target that would more effectively simulate the top twig of a tree. Please let me know if there is any flawed logic.

50ft – 100ft: reduce by 0.9ft
100ft – 150ft: reduce by 1.4ft
150ft – 200ft: reduce by 1.8ft
200ft – 250ft: reduce by 2.2ft
250ft – 300ft: reduce by 2.7ft

Click-over Point (ft) Tape Measure (ft) Difference (ft)
60 59.4 0.6
75 74.1 0.9
90 88.9 1.1
0.9 Average

105 104 1
120 118.4 1.6
135 133.9 1.1
150 148 2
1.4 Average

165 163.4 1.6
180 178 2
195 193.2 1.8
1.8 Average

210 208.2 1.8
225 222.8 2.2
240 237.9 2.1
255 252.3 2.7
2.2 Average

270 267.4 2.6
285 282.3 2.7
300 297.2 2.8
2.7 Average

Thanks,
Matt

[Matt Markworth]

I read a post by Ed saying that the eye is a better point for calibration and I realized that I was finding the click-over point using the end of the LRF, instead of the eyepiece. This makes sense because that’s where the angle measurement is taken. I adjusted my reduction by .3ft to account for this.

50ft – 100ft: reduce by 0.6ft
100ft – 150ft: reduce by 1.1ft
150ft – 200ft: reduce by 1.5ft
200ft – 250ft: reduce by 1.9ft
250ft – 300ft: reduce by 2.4ft

Matt

[edfrank]

Matt,

Those corrections seem reasonable to me. Just do you measurements in the field and rough calculations, then do the corrections all at once at the end of the day. That way you do not forget to correct some, while you do corrections on the others. Bob Leverett is the man on calibration and instrument error. Everything you have done is correct as far as I can see.

Ed

[pdbrandt]

This is a great thread. Thanks, Matt and all for the descriptions - especially how to correctly calibrate a Nikon 440. I bought one on eBay and it will arrive latter this week.