please refer to fig1 . fig1 is a flowchart of a preferred embodiment according to the present invention . as shown in drawing , the present invention utilizes the following steps : an environment input step ( s 101 ), a detection point calculation step ( s 102 ), a measuring step ( s 103 ), an assumed non - detected position point step ( s 104 ), a portable device signal measuring step ( s 105 ), a position point calculation step ( s 106 ), a positioning modification step ( s 107 ), a service integration step ( s 108 ), and a signal feed back step ( s 109 ). please refer to fig2 . fig2 is a schematic drawing of a step for calculating detection points for the preferred embodiment of the present invention . in the environment input step ( s 101 ), a location - based system divides an entire network into a plurality of position points , for example 36 position points , setting an access point as position points 201 , 202 , 203 , 204 , and an obstacle position points 21 . in the detection point calculation step ( s 102 ), the system utilizes a detection algorithm to obtain a position point that needs to be measured . please refer to fig4 . fig4 is a flowchart of the detection algorithm in the preferred embodiment according to the present invention . as shown in drawing , the detection algorithm comprises : an detection point ratio input step ( s 401 ), a detection point selection step ( s 402 ), a detection point suggestion step for obstacles ( s 403 ), a detection point suggestion step for a first area ( s 404 ), a detection point suggestion step for a second area ( s 405 ), and a random detection point suggestion step ( s 406 ). in the detection point ratio input step ( s 401 ), a detection point ratio is input to provide a position point number that indicates how many position points need to be measured for their respective signal strengths ; for example , if the ratio is 1 : 2 , then for 36 position points , at least 18 position points need to be actually measured . in the detection point selection step ( s 402 ), a few selected detection points 22 are set , which are usually points that are difficult to measure , such as a position point 221 among obstacles . in the detection point suggestion step for obstacles ( s 403 ), all position points 231 around the obstacle position points 21 are set as suggested detection points 23 . in the detection point suggestion step for a first area ( s 404 ), all position points which are not obstacles 21 , or other detection points ( including the selected detection points 22 and the suggested detection points 23 ) are sequentially selected ; if a 3 × 3 surrounding area has within it no obstacle and other detection point ( including the selected detection points 22 and the suggested detection points 23 ), this position point is set as a suggested point 232 , as shown in fig2 . in a detection points suggestion step for a second area ( s 405 ), all position points which are not obstacles 21 or other detection points ( including the selected detection points 22 and the suggested detection points 23 ) are sequentially selected , if within their 3 × 3 surrounding area there are no obstacle 21 and other detection points ( including the selected detection points 22 and the suggested detection points 23 ), then this position point is set as a suggestion point 23 . in the random detection point suggestion step ( s 406 ), a ratio of the number of all detection points ( including the selected detection points 22 and the suggested detection points 23 ), and the number of all position points , is compared with the input detection point ratio ; for example , in fig2 , there is 1 selected detection point 22 , and 12 suggested detection points 23 , which does not match the predetermined detection point ratio of 1 : 2 , and so the other requested five selected detection points 22 are randomly selected from other points that are not obstacles 21 and detection points 22 , 23 by the system and set as selected detection points 222 . please refer again to fig1 and fig2 . in the measuring step ( s 103 ), a portable device can be utilized to measure the signal strengths separately from four access points 201 , 202 , 203 , 204 at all selected detection points 22 , and the suggested detection points 23 , when facing front , back , left and right , and a relational list between the positioning signal strengths and the position points is built up . in fig3 , the assumed non - detected position point step utilizes the formula to obtain positioning signal strengths from four directions , and adds the positioning signal strengths into the relational list ; wherein , d is a distance between the positioning point and an access point , d 0 is a distance between a signal obtained point and the access point , waf is an obstacle fading factor , n is a signal fading factor , nw is the number of obstacles between the positioning point and the access point , and p ( d 0 ) is the signal strength of the signal obtained point . afterwards , all four directions of the positioning signal strengths of every position point in the system are added into the relational list to provide a complete positioning structure . the above - mentioned factor nw is the number of obstacles between the positioning points and the access points . however , if nw exceeds a predetermined obstacle number threshold value c , the formula changes to : p ⁡ ( d ) = p ⁡ ( d 0 ) - 10 ⁢ n ⁢ ⁢ log ⁡ ( ⅆ ⅆ 0 ) - c × waf , the above - mentioned signal obtained signal points are any selected reference position points located on a link between the positioning point and the access point . the factor p ( d 0 ) is the positioning signal strength , and the reference points are usually based upon the access point , so that d 0 = 1 . please refer again to fig1 and fig3 . in the portable device signal measuring step ( s 105 ), the portable device that needs position information measures from four directions the position signal strengths of four access points 201 , 202 , 203 , 204 at its current position and sends these signal strengths back to the system . in the position point calculation step , the system compares the position signal strengths with the position signal strengths in the relational list and obtains several matching position points and a most possible position point according a viterbi algorithm . regarding the viterbi algorithm , please refer to fig5 . a position point 501 is a previous position point , a position point 502 is an even more previous position point ; these two points are connected via a directional line , and an orthogonal straight line 511 perpendicular to the directional line is added at the position point 501 . similarly , an n times previous position point 503 and the position point 501 can be connected by another directional line and another corresponding orthogonal line 512 can also be obtained . the line 511 and the line 512 use the position point 501 as a center to generate four areas : area 521 , area 522 , area 523 and area 524 . a human behavior model is utilized , which assumes that the chance of moving forward is larger than the chance of turning , and even larger than the chance of moving backwards ; therefore , assuming the chance of moving from the position point 502 to the position point 501 is a ( a & gt ; 0 . 5 ), and the chance of moving from the position point 503 to the position point 501 is b ( b & gt ; 0 . 5 ), the most possible position point will be located in area 521 , with a probability of a × b . the probabilities of the area 522 and the area 523 are ( 1 − a )× b and a ×( 1 − b ), while the area 524 has the lowest probability of ( 1 − a )×( 1 − b ). furthermore , in the positioning modification step , a modification model ( such as human movement inertia , movement speed and previous characteristics , etc .) is utilized to modify the obtained possible position point . the service integration step ( s 108 ), activates a back - end application service ( such as message service or other interactive service provided by another program ) according to the corresponding positioning information . in the signal feed back step ( s 109 ), the corresponding position information and the back - end application service are sent to the portable device to provide the information to the user via a network . although the present invention has been explained in relation to its preferred embodiment , it is to be understood that many other possible modifications and variations can be made without departing from the spirit and scope of the invention as hereinafter claimed .