"level of accuracy" starts with the data collection process. This "accuracy" is independent from the GIS and has everything to do with the ability to "measure" the location of things (features). Within the GIS the concern is to properly represent the "accuracy".
A "flaw" in mapinfo is the proper representation of coordinate values. To maintain the proper representation of data accuracy it is necessary to "fix" or set the precision of the numeric representation of coordinates. What you want to understand is the systems ability to maintain the proper representation of coordinate values. More often than not this is ignored in MapInfo. Mapinfo needs a more robust method to handle the representation of coordinate values (MapInfo is not alone, many systems suffer similar problems). If the accuracy of your data is decimal degrees to 3 decimal places of accuracy it should never be represented with 4 or more decimal places. In other words you need to "fix" the decimal representation of the coordinates. Internally the system can use more decimal places (floating point) for calculations so that proper rounding can occur. But the retuned / stored values should conform to the original accuracy level - 3 decimal places in this case. All the systems on the market handle this a little differently. All the systems require a some "manual" work to properly fix the decimal representation. There have been some other discussions about this on this site. Things have evolved in the area of accuracy representation and standards for the surveying community. There is need for a distinction between base and line length error. A "traverse survey" will use a measure of accuracy, or precision, represented as parts per million (1st order survey accuracy being 1 part per million. Which means that from a point of beginning the surveyor is ensuring that there is only 1' of error in every million feet of a traverse). There are various degrees of accuracy - you can search these out at sites like ACSM.NET or BLM.GOV With the establishment of good control networks and high accuracy GPS capabilities there is a "BASE" measure of accuracy represented in mm. Meaning that the location of the feature is within X mm of its true location with 95% confidence (meaning that this is true 95 out of 100 times). In most circumstances you are concerned with the BASE spatial accuracy of the coordinates. You also need to know about the data you are collecting. Determining and representing the location of a marine navigation device (lighthouse, buoy, etc.) is different from representing the boundary of specific marine feature such as a habitat that might have a "fuzzy" boundary. Enter MetaData... You should always consult the metadata. The spatial accuracy of the data should be clearly stated in the metadata, along with other details that describe the overall confidence and "usefulness" of the data. -----Original Message----- From: Stanley K. Johnston [mailto:[EMAIL PROTECTED] Sent: Wednesday, March 03, 2004 07:09 PM To: [EMAIL PROTECTED] Subject: MI-L Surveying level accuracy MapInfo vs. ESRI vs. others Does anyone know the level of accuracy of MapInfo vs. ESRI or other GIS systems when it comes to surveying, specifically in the marine environment? Stan Johnston --------------------------------------------------------------------- List hosting provided by Directions Magazine | www.directionsmag.com | To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, e-mail: [EMAIL PROTECTED] Message number: 10696 --------------------------------------------------------------------- List hosting provided by Directions Magazine | www.directionsmag.com | To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, e-mail: [EMAIL PROTECTED] Message number: 10731
