Determinng sample size for Positional Accuracy Assesment Tool

Question

I am trying to evaluate the accuracy of some vetor layers against a reference image using the PAAT tool.   Does anyone know how to determine the sample needed? For example: I want to evaluate the roads feature class with 700 segments. How many points should I collect?

Thanks,

Ovidio

michellej · Answer

Ovidio,

Yes, the instructions from Pete is for version 10.  I hope it works out for you when you upgrade.

Anyhow, I wanted you to know that your question prompted the Data Reviewer team, with the help from Pete, to post a blog answering your question.  You can check it out here.

Thank you for your forum question!

OvidioRivero · Answer

Pete,

Thanks for the instructions. Unfortunately the steps you describe seem to be for ArcGIS 10. I have 9.3.1. The Select data check drop down has nothing about sampling.  There is an icon for sampling on the toolbar but when you select it it expects you to give it a number of features or a precentage and optionally a sample table. I read the online documentation for 10   and the steps seem to be the same. I am getting it in a couple of weeks and I'l try it again.

Thanks,

PeterAniello · Answer

Pete,

What I want to know is how to determine the sample size(10%, 20% etc.). The tool you mention is good because it gives you the locations of your sample features but it expects you to know what sample size to use. I have run the PAAT on a 2% sample and a 10% sample of features set up 90% confidence and it gives me  the same result. Should I use 2% to finish my assesment faster or should I stick to a bigger sample?

Thanks,

Ovidio

Ovidio, to generate the sample size, use the Auto Calculate function in the Data Reviewer Sample Check (Data Reviewer --> Select Data Check --> Advanced Checks --> Sampling Check).  The Auto Calculate function lets you generate a statistically significant sample size by specifying a confidence interval and a margin of error on the confidence interval.  Once the tool generates the sample size, you can then use the Browse Results dialog in Data Reviewer to visit the features and read the locations of the features with the PAAT.  Alternatively, once the Sampling Check has generated the sample size, you can set up the PAAT's grid with a number of grid cells equal to the sample size.  Either method will work.

Pete